Gauges

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Oil Pressure

An oil pressure gauge can give you an excellent indication of the health of various systems in your engine. The key is to establish baseline readings when your engine is healthy, and then be aware of any changes you see over time. The Lubrication System The oil pump takes in oil from the sump (oil pan), and forces it through the engine under pressure. Since the oil pump is driven (indirectly) by the crankshaft, oil pressure is to some extent determined by engine RPM. Pressure is kept from rising too high by a relief valve. Typically, this is a spring-loaded ball, where a predetermined amount of pressure lifts the ball off its seat to allow oil to return to the oil pan without circulating through the engine.

The oil is pumped through drillings in the block and head, lubricating the bearings and also helping cool the engine. After it emerges from the drillings in the crankshaft and other parts, it drains back (with virtually no pressure) into the sump to be re-circulated. A filter in the circuit keeps dirt and metal filings from scratching the bearings or damaging the oil pump.

What Determines Oil Pressure? At lower engine speeds, oil pressure is limited by the clearances between the various bearings and journals. For example, as the space between the crankshaft bearing journals and its bearings increases through wear, oil pressure will be lower because oil can flow out of the space more easily. The same is true for the journals on the big ends of the connecting rods. Thus, everything else being equal, low oil pressure can indicate worn bearings.

There are other factors that affect oil pressure, though. When the oil is colder, it has a higher viscosity (it's thicker), which means it cannot slip through the bearing clearances as easily. You'll notice that oil pressure at idle is quite a bit higher when the engine is first started up. On some cars (Fiats, for example), low oil pressure at idle when the engine is hot is quite normal. Oil flow may be perfectly adequate, even though the pressure is low.

It stands to reason that thinner (lower-weight) oil will indicate lower oil pressure than a thicker oil, at least at idle and moderate engine speeds. Lower pressure caused by changing to a lower-viscosity oil may not indicate a problem, provided it is not being overheated. If the oil is thinner because it is breaking down, too hot, or diluted with gasoline from an over-rich mixture or worn rings, you should change oil at once and correct the problem.

Potential Causes of Low Oil Pressure: Low Oil Level - You may first notice your oil pressure dip during a hard corner or under sharp braking. Stop at once and top up the oil, or you can seriously damage your engine! Diluted or Worn-Out Oil - (see above). Damaged Oil Pan or Pickup Tube - Have you scraped or banged your oil pan? Stop at once! High Oil Temperature - Generally not a big factor, but if you're pulling a trailer or running flat out in really hot weather, your oil can run well over 250 degrees F., and oil pressure will be lower. Worn Engine Bearings - (see above). A further indication can be a heavy knocking under engine load (main bearings) or a lighter knocking (connecting rod bearings). Worn Oil Pump - This could be anything from a slight reduction all the way to catastrophic failure (which is rare unless the pump has ingested bits of metal from some other failure). Dropped Crankshaft Plug(s) - This is not terribly uncommon in 4-cylinder Alfa Romeo engines. These metal plugs fill the holes where the factory drilled oil passages in the crankshaft. If one falls out, oil pressure will suddenly drop across the board. You can still drive (slowly) to get home, but the plug(s) will need to be replaced.

What About High Oil Pressure? High oil pressure is not generally a concern, but if pressure suddenly increases, there may be a problem with the pressure relief valve. Switching to a higher-viscosity oil will also show higher readings. In choosing oil weight, it's best to follow the manufacturer's recommendations for the season and type of driving.

Exhaust Temperature

An exhaust gas temperature gauge (EGT gauge) is used to monitor the exhaust gas temperature of an internal combustion engine in conjunction with a thermocouple-type pyrometer. By monitoring EGT, the driver can get an idea of the car's air-fuel ratio. At a balanced air-fuel ratio, the exhaust gas temperature is lower than that in a lean or rich air-fuel ratio. High temperatures (typically above 870 degrees C) can be an indicator of dangerous conditions that can lead to catastrophic engine failure.

This meter is most used in tuning turbo-equipped cars. If the sensor is installed at the manifold collector before the turbo, the turbine inlet temperature can be monitored. If the sensor is installed after the turbo, the exhaust temperature can be monitored. Because EGT will typically drop 100-150 degrees C across the turbine, installers in general will try to put the thermocouple as close to the cylinder head as possible to give a true reading that will require less mental math to read properly, and a reading that will react faster to the engine's condition compared to an installation after the turbo.

Using an EGT alone is considered an older technique for getting the most out of an engine, as a gauge-type wideband digital oxygen sensor can be purchased for about the same price, or for a little more. However, some advanced racers will use EGT gauges in combination with a wideband oxygen sensor to 'lean' the fuel ratio a bit to safely raise the temperature for more power.

Fuel Pressure

Intake Manifold Pressure

Oil Pressure

Oil Temperature

Tachometer

Vacuum

Though little understood, the vacuum gauge is probably the best single indicator of your engine's health you can get.

When your car is idling-whether it's fuel injected or carbureted-the throttle plate or plates are restricting the amount of air the engine can breathe in. The pistons are attempting to "suck" the mixture past the throttle. (Of course, in reality, it is atmospheric pressure that is attempting to "push" air into the engine as the pistons travel downward on their intake strokes.) When throttle is closed, vacuum is high in the intake manifold, from the throttle plate(s) to the combustion chambers. By contrast, at wide open throttle there is relatively little restriction to outside air entering the intake manifold, so vacuum in the manifold is very low.

A vacuum gauge reads pressure differences from atmospheric pressure, so the reading is zero in our "normal" sea of air. By convention, vacuum gauges in the US read "inches of Mercury."

Unlike a fuel gauge, the vacuum gauge will keep you entertained with its instantaneous, wide-ranging movements.

When you floor the accelerator pedal, you can watch manifold pressure (another word for vacuum) swing from strongly negative to nearly zero (atmospheric pressure). When your engine is "on the overrun," like using engine braking down a steep hill at high RPM, you'll see really high vacuum readings. Naturally, turbocharged and supercharged will show very different results, with readings swinging into the positive at high speed. IAP's vacuum gauge is not designed for turbo or supercharged vehicles. Your vacuum gauge is also a sort of "poor man's" fuel mileage indicator; when vacuum is low, you are burning more fuel.

Absolute readings are not as useful as changes over time. That is, if you establish baseline readings under a variety of circumstances, you will know what to look for if your engine begins to deviate. Everything else aside, a high vacuum reading tends to indicate a healthy engine.

Having said this, we can make generalities about the readings you can expect. Note that engines with performance camshafts tend to read lower vacuum. Readings are also lower at higher altitudes; the rule of thumb is approximately 1 inch of mercury for every 1,000 feet of altitude gain.

The following readings will not apply to turbocharged engines, or cars with a separate venturi for each cylinder (like Weber DCOE or Dellorto carbs). All readings are inches of mercury (in. Hg.).

ENGINE STATE VACUUM GAUGE READING INDICATION Steady idle (800-1200rpm) Gauge steady, 17-22 Normal & healthy Steady idle (800-1200 rpm) Intermittently drops several needle divisions Sticking valve or broken valve spring Steady idle (800-1200 rpm) Steady, low reading, 8-14 Small vacuum leak or valve timing off; could have low compression/worn rings (verify with a compression or leakdown test). Steady idle (800-1200 rpm) Steady, low reading, under 8 Vacuum leak (check brake booster, vacuum lines, etc.) Idle (800-1200 rpm) Needle drops sharply on a regular rhythm Burnt valve, or a valve with clearance too tight Idle (800-1200 rpm) Needle drifts up & down, along with rpm drift Mixture off or small vacuum leak Idle (800-1200rpm) Vacuum gradually drops Excessive exhaust back pressure (plugged muffler or catalytic converter) Idle (800-1200rpm) Intermittent fluctuation Ignition miss; sticking valve Idle (800-1200rpm) Steady, above 22 Ignition timing may be too advanced Open & close throttle quickly Drops to about 2, jumps to about 25 Healthy engine Open & close throttle quickly Drops to 0, jumps to about 20 May confirm worn rings (especially if idle shows only about 15-20) Verify with a compression or leakdown test. http://www.international-auto.com/fiat-lancia-tips-on-reading-gauges/tips-on-reading-gauges-vacuum-gauges.cfm


Water Temperature

Voltmeter

The voltmeter will let you know if there is a problem with your charging system or battery. It is easier to use (and especially to install) than an ammeter. Learn your car's baseline voltages, so you can recognize when something may be going wrong. Here are a few possible voltmeter indications, assuming the car has been running enough to charge the battery. All accessories should be off:

VEHICLE STATE VOLTMETER READING INDICATION Engine not running Engine off voltage is at least 12V So far, so good. Note voltage reading. Engine not running Engine off voltage is less than 12V Battery run down. Charge before continuing tests. Cars with electronic ignition may not start, even if engine cranks. During Cranking Voltage should be above 9.5V If not, battery may not have enough capacity (too small or weakened by sulfanation or "old age.") Increase engine to 1500rpm Voltage exceeds engine-off voltage by more than 2V Probable over-charging; look for bad regulator or regulator ground, or wiring problem. Increase engine to 1500rpm Voltage exceeds engine-off voltage by less than 2V Proceed to Load Test Load Test: 3000rpm with all lights, heater & radio on Voltage exceeds engine-off voltage by less than 1/2 Volt Charging system may be faulty Load Test: 3000rpm with all lights, heater & radio on Voltage exceeds engine-off voltage by more than 1/2 Volt Charging system ok Any rpm Voltage regularly exceeds 14.5V. (May also blow lamp bulbs frequently, and/or tend to make battery run dry.) Probable over-charging; look for bad regulator or regulator ground, or wiring problem. http://www.international-auto.com/fiat-lancia-tips-on-reading-gauges/tips-on-reading-gauges-voltmeter.cfm


Air/Fuel Ratio

The A/F Ratio Meter is a voltmeter with a range of 0 to 1 Volt. The meter displays the output voltage of the vehicles oxygen sensor through 20 LED's. The first LED will come on at a voltage of .050V, the second at .100V, the third at .150V, etc.

LEAN RANGE: Four red LED's (.050 to .249V) STOICHIOMETRIC RANGE: Ten yellow LED's (.250 to .749V) RICH RANGE: Six green LED's (.750 to 1.000V)

The stoichiometric air/fuel ratio is the chemically correct ratio, theoretically all of the oxygen and all of the fuel are consumed. The mixture is neither rich nor lean. However, due to the fact that combustion is never perfect in the real world, there will always be a small amount of oxygen left in the exhaust. This small amount that is left is what the oxygen sensor measures. The smaller the amount of oxygen that is left in the exhaust, the richer the A/F ratio is, and the higher the oxygen sensor voltage is. The on-board computer or Powertrain Control Module (PCM) monitors the voltage from the oxygen sensor. If the PCM sees an oxygen sensor voltage greater than .450V, it immediately starts to reduce the amount of fuel that is metered into the engine by reducing the on time to the fuel injectors. When this happens, the A/F ratio starts to go in the lean direction, and the oxygen sensor voltage starts to go down. When the voltage drops below .450V, the PCM immediately starts to increase the fuel metered to the engine by increasing the on time to the fuel injectors to produce a richer A/F ratio. This occurs until the oxygen sensor voltage goes above .450V. This repeating cycle happens very fast (many times per second). The PCM is said to be in closed loop. It is constantly monitoring the oxygen sensor voltage and adjusting the on time of the fuel injectors to maintain a stoichiometric A/F ratio. This A/F ratio produces the lowest harmful exhaust emissions, and allows the catalytic converter to operate at peak efficiency, therefore reducing the exhaust emissions further.

Since the oxygen sensor output is non-linear and very sensitive at the stoichiometric A/F ratio it will cause the A/F meter LED's to bounce back and forth rapidly. A very small change in A/F ratio causes a large change in oxygen sensor voltage as can be seen on the graph. This causes the A/F ratio meter LED's to rapidly cycle back and forth, and is normal operation when the PCM is in closed loop and trying to maintain a stoichiometric A/F ratio. The oxygen sensor is very accurate at indicating a stoichiometric A/F ratio. It is also very accurate at indicating an A/F ratio that is richer or leaner than stoichiometric. However it can not indicate what exactly the A/F ratio is in the rich and lean areas due to the fact that the oxygen sensor output changes with the oxygen sensor temperature and wear. As the sensor temperature increases, the voltage output will decrease for a given A/F ratio in the rich area, and increase in the lean area as shown on the graph.

During wide open throttle (throttle opening greater than 80% as indicated by the throttle position sensor) the A/F ratio will be forced rich by the PCM for maximum power. During this time the oxygen sensor outputs a voltage that corresponds to a rich A/F ratio. But the PCM ignores the oxygen sensor signal because it is not accurate for indicating exactly what the A/F ratio is in this range. The PCM is now in open loop, and relies on factory programmed maps to calculate what the on time of the fuel injectors should be to provide a rich A/F ratio for maximum power. The A/F ratio meter should indicate rich during this time.

During hard deceleration the PCM will command an extremely lean mixture for lowest exhaust emissions. This may cause the A/F ratio meter not to indicate anything. The A/F ratio is so lean that it is outside the range that the meter will indicate.

A bit of clarification on how they work and what they look like when they are working.

A couple of things to look for when connecting your A/F ratio gauge to your vehicle are the O2 sensor itself, and it's current condition. Normally, an oxygen sensor is designed to last about 50,000 miles. However, its life can be shortened by contamination, blocked outside air, short circuits, and/or poor electrical connections.

The oxygen sensor can become contaminated by, but not limited to the following:

Also keep in mind that the O2 sensor only generates anywhere from 0-1 volt, and averages around .5 volts. A poor or deteriorating electrical connection is not only frustrating but could also prevent this small voltage from reaching the computer and the gauge, too. Always be sure to check the electrical connections as well.

Testing the O2 sensor output is another way to ensure proper operation. A digital voltmeter can be used to test the output of an O2 sensor. WARNING! Be sure to only use a high impedance digital multimeter to measure the O2 sensor voltages. A conventional analog or low resistance meter can draw too much current and potentially damage the sensor itself. So, for testing, be sure to warm the engine to full operating temperature to shift the computer into closed loop. The sensor must be hot(at least 600 degrees or hotter) to operate properly. You may have to warm the engine at fast idle for 5-10 minutes with some cars to reach the target temp. Note that a few systems can drop out of closed loop at idle, so you may have trouble keeping the engine at a hot enough temp to see the readings you are looking for.

Airfuel chart1.gif

Crankshaft position sensor

A crank position sensor is an electronic device used in an internal combustion engine to monitor the position or rotational speed of the crankshaft. This information is used by engine management systems to control ignition system timing and other engine parameters. Before electronic crank sensors were available, the distributor would have to be manually adjusted to a timing mark on the engine.

The crank sensor can be used in combination with a similar camshaft position sensor to monitor the relationship between the pistons and valves in the engine, which is particularly important in engines with variable valve timing. This method is also used to "syncronise" a four stroke engine upon starting, allowing the management system to know when to inject the fuel. It is also commonly used as the primary source for the measurement of engine speed in revolutions per minute.

Common mounting locations include the main crank pulley, the flywheel, or on the crankshaft itself. This sensor is the most important sensor in modern day engines. When it fails, there is a chance the engine will not start, or cut out while running.

Commonly a hall effect sensor is used, which is placed adjacant to a spinning ferrous disk

Some engines, such as GM Premium V engine, use crank position sensors which read a reluctor ring integral to the harmonic balancer. This is a much more accurate method of determining the position of the crankshaft, and allows the computer to determine within a few degrees the exact position of the crankshaft (and thereby all connected components) at any given time.

The functional objective for the crankshaft position sensor is to determine the position and/or rotational speed (RPM) of the crank. Engine Control Unit's use the information transmitted by the sensor to control parameters such as ignition timing and fuel injection timing. The sensor output can also be related to other sensor data including the cam position to derive the current combustion cycle, this is very important for the starting of a four stroke engine.

The rotating disc of the sensor system does not necessarily have to be mounted directly on the crankshaft but can also be driven by a chain or toothed belt off the crankshaft as is the case in some Hondas where the crank position sensor is located in the distributor, effectively driven by the timing belt. Other locations include: The front of the crank on the engine block below or next to the water pump, on the main crank pulley or near the flywheel..

Sometimes the sensor may become burnt or worn out. The most likely causes of crankshaft position sensor failure are exposure to extreme heat when you have a gasket or crank problem, corruption from oil or other engine fluids that leaked onto the sensor, or wear over a long period of time. When it goes bad, it stops transmitting the signal which contains the vital data for the ignition and other parts in the system.

A bad crank position sensor can worsen the way the engine idles, the pistons fire, or the acceleration behavior. If the engine is revved up with a bad or faulty sensor, it may cause misfiring, motor vibration or backfires. Accelerating might be hesitant, and abnormal shaking during engine idle might occur. In the worst case the car may not start.

Engine coolant temperature sensor

The Engine Coolant Temperature Sensor (ECT) tells an automobile's ECU what the engine temperature is, so that optimum driveability is realized while the engine is warming up and when the engine has reached operating temperature.

Exhaust gas temperature gauge

MAP sensor

The Manifold Absolute Pressure sensor (MAP sensor) is one of the sensors used in an internal combustion engine's electronic control system. Engines that use a MAP sensor are typically fuel injected. The manifold absolute pressure sensor provides instantaneous manifold pressure information to the engine's electronic control unit (ECU). The data is used to calculate air density and determine the engine's air mass flow rate, which in turn determines the required fuel metering for optimum combustion. A fuel-injected engine may alternately use a MAF (mass air flow) sensor to detect the intake airflow. A typical configuration employs one or the other, but seldom both.

MAP sensor data can be converted to air mass data using the speed-density method. Engine speed (RPM) and air temperature are also necessary to complete the speed-density calculation. The MAP sensor can also be used in on-board diagnostics applications to test the EGR (exhaust gas recirculation) valve for functionality, an application typical in OBD II equipped General Motors engines.

The following example assumes the same engine speed and air temperature.

  • Condition 1:
An engine operating at WOT (wide open throttle) on top of a very high mountain has a MAP of about 15" Hg or 50 kPa (essentially equal to the barometer at that high altitude).
  • Condition 2:
The same engine at sea level will achieve 15" Hg of MAP at less than WOT due to the higher barometric pressure.

The engine requires the same mass of fuel in both conditions because the mass of air entering the cylinders is the same.

If the throttle is opened all the way in condition 2, the manifold absolute pressure will increase from 15" Hg to nearly 30" Hg (~100 kPa), about equal to the local barometer, which in condition 2 is sea level. The higher absolute pressure in the intake manifold increases the air's density, and in turn more fuel can be burned resulting in higher output.

Almost anyone who has driven up a high mountain is familiar with the reduction in engine output as altitude increases.

Vacuum is the difference between the absolute pressures of the intake manifold and atmosphere. Vacuum is a "gauge" pressure, since gauges by nature measure a pressure difference, not an absolute pressure. The engine fundamentally responds to air mass, not vacuum, and absolute pressure is necessary to calculate mass. The mass of air entering the engine is directly proportional to the air density, which is proportional to the absolute pressure, and inversely proportional to the absolute temperature.

MAP sensors measure absolute pressure. Boost sensors or gauges measure the amount of pressure above a set absolute pressure. That set absolute pressure is usually 1 atmosphere (1 atm) or 14.7 psi. This is commonly referred to as gauge pressure. So boost pressure is relative to absolute pressure - as one increases or decreases, so does the other. It is a one-to-one relationship with an offset of -14.7 psi for boost pressure. Thus a MAP sensor will always read 14.7 psi more than a boost sensor measuring the same conditions. A MAP sensor will never read negative because it is measuring absolute pressure, where zero is the total absence of pressure. (It is possible to have conditions where negative absolute pressure can be observed, but none of those conditions occur in the air intake of an internal combustion engine.) Boost sensors can read negative numbers, indicating vacuum or suction (a condition of lower pressure than the surrounding atmosphere). In forced induction engines (supercharged or turbocharged), a negative boost reading indicates that the engine is drawing air faster than it is being supplied, creating suction. This is often called vacuum pressure when referring to internal combustion engines.

In short: most boost sensors will read 14.7 psi less than a MAP sensor reads. One can convert boost to MAP by adding 14.7 psi. One can convert from MAP to boost by subtracting 14.7 psi.


Mass flow sensor

A mass air flow sensor is used to find out the mass flowrate of air entering a fuel-injected internal combustion engine. The air mass information is necessary for the engine control unit (ECU) to balance and deliver the correct fuel mass to the engine. Air changes its density as it expands and contracts with temperature and pressure. In automotive applications, air density varies with the ambient temperature, altitude and the use of forced induction, which means that mass flow sensors are more appropriate than volumetric flow sensors for determining the quantity of intake air in each piston stroke.

There are two common types of mass airflow sensors in use on automotive engines. These are the vane meter and the hot wire. Neither design employs technology that measures air mass directly. However, with additional sensors and inputs, an engine's electronic control unit can determine the mass flowrate of intake air.

Both approaches are used almost exclusively on electronic fuel injection (EFI) engines. Both sensor designs output a 0.0–5.0 volt or a pulse-width modulation (PWM) signal that is proportional to the air mass flow rate, and both sensors have an intake air temperature (IAT) sensor incorporated into their housings.

When a MAF is used in conjunction with an oxygen sensor, the engine's air/fuel ratio can be controlled very accurately. The MAF sensor provides the open-loop controller predicted air flow information (the measured air flow) to the ECU, and the oxygen sensor provides closed-loop feedback in order to make minor corrections to the predicted air mass. Also see MAP sensor.

A hot wire mass airflow sensor determines the mass of air flowing into the engine’s air intake system. The General Motors division (GM) was the first car company to use the hot wire sensor. This is achieved by heating a wire with an electric current that is suspended in the engine’s air stream, like a toaster wire. The wire's electrical resistance increases as the wire’s temperature increases, which limits electrical current flowing through the circuit. When air flows past the wire, the wire cools, decreasing its resistance, which in turn allows more current to flow through the circuit. As more current flows, the wire’s temperature increases until the resistance reaches equilibrium again. The amount of current required to maintain the wire’s temperature is directly proportional to the mass of air flowing past the wire. The integrated electronic circuit converts the measurement of current into a voltage signal which is sent to the ECU.

If air density increases due to pressure increase or temperature drop, but the air volume remains constant, the denser air will remove more heat from the wire indicating a higher mass airflow. Unlike the vane meter's paddle sensing element, the hot wire responds directly to air density. This sensor's capabilities are well suited to support the gasoline combustion process which fundamentally responds to air mass, not air volume.

This sensor sometimes employs a mixture screw, but this screw is fully electronic and uses a variable resistor (potentiometer) instead of an air bypass screw. The screw needs more turns to achieve the desired results. A hot wire burn-off cleaning circuit is employed on some of these sensors. A burn-off relay applies a high current through the platinum hot wire after the vehicle is turned off for a second or so, thereby burning or vaporizing any contaminants that have stuck to the platinum hot wire element.

The hot film MAF sensor works somewhat similar to the hot wire MAF sensor, but instead it usually outputs a frequency signal. This sensor uses a hot film-grid instead of a hot wire. It is commonly found in late 80’s early 90’s fuel injected vehicles. The output frequency is directly proportional to the amount of air entering the engine. So as air flow increases so does frequency. These sensors tend to cause intermittent problems due to internal electrical failures. The use of an oscilloscope is strongly recommended to check the output frequency of these sensors. Frequency distortion is also common when the sensor starts to fail. Many technicians in the field use a tap test with very conclusive results. Not all HFM systems output a frequency. In some cases, this sensor works by outputting a regular varying voltage signal.

Some of the benefits of a hot-wire MAF compared to the older style vane meter are:

  • responds very quickly to changes in air flow
  • low airflow restriction
  • smaller overall package
  • less sensitive to mounting location and orientation
  • no moving parts improve its durability
  • less expensive
  • separate temperature and pressure sensors are not required (to determine air mass)

There are some drawbacks:

  • dirt and oil can contaminate the hot-wire deteriorating its accuracy
  • installation requires a laminar flow across the hot-wire


Oxygen sensor

An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman. The original sensing element is made with a thimble-shaped zirconia ceramic coated on both the exhaust and reference sides with a thin layer of platinum and comes in both heated and unheated forms. The planar-style sensor entered the market in 1998 (also pioneered by Bosch) and significantly reduced the mass of the ceramic sensing element as well as incorporating the heater within the ceramic structure. This resulted in a sensor that started sooner and responded faster. The most common application is to measure the exhaust gas concentration of oxygen for internal combustion engines in automobiles and other vehicles.

Automotive oxygen sensors, colloquially known as O2 sensors, make modern electronic fuel injection and emission control possible. They help determine, in real time, if the air fuel ratio of a combustion engine is rich or lean. Since oxygen sensors are located in the exhaust stream, they do not directly measure the air or the fuel entering the engine. But when information from oxygen sensors is coupled with information from other sources, it can be used to indirectly determine the air-to-fuel ratio. Closed-loop feedback-controlled fuel injection varies the fuel injector output according to real-time sensor data rather than operating with a predetermined (open-loop) fuel map. In addition to enabling electronic fuel injection to work efficiently, this emissions control technique can reduce the amounts of both unburnt fuel and oxides of nitrogen entering the atmosphere. Unburnt fuel is pollution in the form of air-borne hydrocarbons, while oxides of nitrogen (NOx gases) are a result of combustion chamber temperatures exceeding 1,300 kelvin due to excess air in the fuel mixture and contribute to smog and acid rain. Volvo was the first automobile manufacturer to employ this technology in the late 1970s, along with the 3-way catalyst used in the catalytic converter.

The sensor does not actually measure oxygen concentration, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in air. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess.

Modern spark-ignited combustion engines use oxygen sensors and catalytic converters in order to reduce exhaust emissions. Information on oxygen concentration is sent to the engine management computer or ECU, which adjusts the amount of fuel injected into the engine to compensate for excess air or excess fuel. The ECU attempts to maintain, on average, a certain air-fuel ratio by interpreting the information it gains from the oxygen sensor. The primary goal is a compromise between power, fuel economy, and emissions, and in most cases is achieved by an air-fuel-ratio close to stoichiometric. For spark-ignition engines (such as those that burn gasoline, as opposed to Diesel, the three types of emissions modern systems are concerned with are: hydrocarbons (which are released when the fuel is not burnt completely, such as when misfiring or running rich), carbon monoxide (which is the result of running slightly rich) and NOx (which dominate when the mixture is lean). Failure of these sensors, either through normal aging, the use of leaded fuels, or fuel contaminated with silicones or silicates, for example, can lead to damage of an automobile's catalytic converter and expensive repairs.

Tampering with or modifying the signal that the oxygen sensor sends to the engine computer can be detrimental to emissions control and can even damage the vehicle. When the engine is under low-load conditions (such as when accelerating very gently, or maintaining a constant speed), it is operating in "closed-loop mode." This refers to a feedback loop between the ECU and the oxygen sensor(s) in which the ECU adjusts the quantity of fuel and expects to see a resulting change in the response of the oxygen sensor. This loop forces the engine to operate both slightly lean and slightly rich on successive loops, as it attempts to maintain a mostly stoichiometric ratio on average. If modifications cause the engine to run moderately lean, there will be a slight increase in fuel economy, sometimes at the expense of increased NOx emissions, much higher exhaust gas temperatures, and sometimes a slight increase in power that can quickly turn into misfires and a drastic loss of power, as well as potential engine damage, at ultra-lean air-to-fuel ratios. If modifications cause the engine to run rich, then there will be a slight increase in power to a point (after which the engine starts flooding from too much unburned fuel), but at the cost of decreased fuel economy, and an increase in unburned hydrocarbons in the exhaust which causes overheating of the catalytic converter. Prolonged operation at rich mixtures can cause catastrophic failure of the catalytic converter (see backfire). The ECU also controls the spark engine timing along with the fuel injector pulse width, so modifications which alter the engine to operate either too lean or too rich may result in inefficient fuel consumption whenever fuel is ignited too soon or too late in the combustion cycle.

When an internal combustion engine is under high load (e.g. wide open throttle), the output of the oxygen sensor is ignored, and the ECU automatically enriches the mixture to protect the engine, as misfires under load are much more likely to cause damage. This is referred to as an engine running in 'open-loop mode'. Any changes in the sensor output will be ignored in this state. In many cars (with the exception of some turbocharged models), inputs from the air flow meter are also ignored, as they might otherwise lower engine performance due to the mixture being too rich or too lean, and increase the risk of engine damage due to detonation if the mixture is too lean.

Lambda probes are used to reduce vehicle emissions by ensuring that engines burn their fuel efficiently and cleanly and it was first used by Volvo and Saab in that year. The sensors were introduced in the US from about 1980, and were required on all models of cars in many countries in Europe in 1993.

By measuring the proportion of oxygen in the remaining exhaust gas, and by knowing the volume and temperature of the air entering the cylinders amongst other things, an ECU can use look-up tables to determine the amount of fuel required to burn at the stoichiometric ratio (14.7:1 air:fuel by mass for gasoline) to ensure complete combustion.

The sensor element is a ceramic cylinder plated inside and out with porous platinum electrodes; the whole assembly is protected by a metal gauze. It operates by measuring the difference in oxygen between the exhaust gas and the external air, and generates a voltage or changes its resistance depending on the difference between the two.

The sensors only work effectively when heated to approximately 316 °C (600 °F), so most newer lambda probes have heating elements encased in the ceramic that bring the ceramic tip up to temperature quickly. Older probes, without heating elements, would eventually be heated by the exhaust, but there is a time lag between when the engine is started and when the components in the exhaust system come to a thermal equilibrium. The length of time required for the exhaust gases to bring the probe to temperature depends on the temperature of the ambient air and the geometry of the exhaust system. Without a heater, the process may take several minutes. There are pollution problems that are attributed to this slow start-up process, including a similar problem with the working temperature of a catalytic converter.

The probe typically has four wires attached to it: two for the lambda output, and two for the heater power, although some automakers use a common ground for the sensor element and heaters, resulting in three wires. Earlier non-electrically-heated sensors had one or two wires..


Throttle position sensor

A throttle position sensor (TPS) is a sensor used to monitor the position of the throttle in an internal combustion engine. The sensor is usually located on the butterfly spindle so that it can directly monitor the position of the throttle valve butterfly.

The sensor is usually a potentiometer, and therefore provides a variable resistance dependent upon the position of the valve (and hence throttle position).

The sensor signal is used by the engine control unit (ECU) as an input to its control system. The ignition timing and fuel injection timing (and potentially other parameters) are altered depending upon the position of the throttle, and also depending on the rate of change of that position. For example, in fuel injected engines, in order to avoid stalling, extra fuel may be injected if the throttle is opened rapidly (mimicking the accelerator pump of carburetor systems).

More advanced forms of the sensor are also used, for example an extra closed throttle position sensor (CTPS) may be employed to indicate that the throttle is completely closed.

Some ECUs also control the throttle position and if that is done the position sensor is utilised in a feedback loop to enable that control.

Related to the TPS are accelerator pedal sensors, which often include a wide open throttle (WOT) sensor. The accelerator pedal sensors are used in "drive by wire" systems, and the most common use of a wide open throttle sensor is for the kick-down function on automatic transmissions.

Modern day sensors are non contact type, wherein a magnet and a Hall effect sensor are used. In the potentiometric type sensors, two metal parts are in contact with each other, while the butterfly valve is turned from zero to WOT, there is a change in the resistance and this change in resistance is given as the input to the ECU.

Non contact type TPS work on the principle of Hall effect, wherein the magnet is the dynamic part which mounted on the butterfly valve spindle and the Hall effect sensor is mounted with the body and is stationary. When the magnet mounted on the spindle which is rotated from zero to WOT, there is a change in the magnetic field for the Hall effect sensor. The change in the magnetic field is sensed by the Hall effect sensor and the voltage generated is given as the input to the ECU. Normally a two pole magnet is used for TPS and the magnet may be of diametrical type, ring type or segment type. The magnet is defined to have a certain magnetic field.


Exhaust Temperature

An exhaust gas temperature gauge (EGT gauge) is an automotive Measuring instrument used to monitor the exhaust gas temperature of an internal combustion engine or rotary engine in conjunction with a thermocouple-type pyrometer. By monitoring EGT, the driver can get an idea of the car's air-fuel ratio. At a balanced air-fuel ratio, the exhaust gas temperature is lower than that in a lean or rich air-fuel ratio. High temperatures (typically above 1600 degrees F) can be an indicator of dangerous conditions that can lead to catastrophic engine failure.

This meter is most used in tuning turbo-equipped cars. If the sensor is installed at the manifold collector before the turbo, the turbine inlet temperature can be monitored. If the sensor is installed after the turbo, the exhaust temperature can be monitored. Because EGT will typically drop 200-300 degrees F across the turbine, installers in general will try to put the thermocouple as close to the cylinder head as possible to give a true reading that will require less mental math to read properly, and a reading that will react faster to the engine's condition compared to an installation after the turbo.

Using an EGT alone is considered an older technique for getting the most out of an engine, as a gauge-type wideband digital oxygen sensor can be purchased for about the same price, or for a little more. However, some advanced racers will use EGT gauges in combination with a wideband oxygen sensor to 'lean' the fuel ratio a bit to safely raise the temperature for more power.


Fuel Pressure

Intake Manifold Pressure

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