The examples and descriptions in this article apply strictly to four-stroke cycle gasoline engines. Other engine types such as diesel, or two-stroke cycle can differ in the exact implementation, but the general ideas still apply.
MAP sensor data can be converted to air mass data by using a second variable coming from an IAT Sensor (intake air temperature sensor). This is called the speed-density method. Engine speed (RPM) is also used to determine where on a look up table to determine fuelling, hence speed-density (engine speed / air density). The MAP sensor can also be used in OBD II (on-board diagnostics) applications to test the EGR (exhaust gas recirculation) valve for functionality, an application typical in OBD II equipped General Motors engines.
Example
The following example assumes the same engine speed and air temperature in a naturally aspirated engine.
Condition 1:
An engine operating at wide open throttle (WOT) on top of a very high mountain has a manifold pressure of about 50 kPa (essentially equal to the barometer at that high altitude).
Condition 2:
The same engine at sea level will achieve that same 50 kPa (7.25 psi, 14.7 inHG) of manifold pressure at less than (before reaching) 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 50 kPa to nearly 100 kPa (14.5 psi, 29.53 inHG), 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.
Another example is varying rpm and engine loads -
Where an engine may have 60kPa of manifold pressure at 1800 rpm in an unloaded condition, introducing load with a further throttle opening will change the final manifold pressure to 100kPa, engine will still be at 1800 rpm but its loading will require a different spark and fueling delivery.
Vacuum comparison
Engine vacuum is the difference between the pressures in the intake manifold and ambient atmospheric pressure. Engine 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.
Note: Carburetors are largely dependent on air volume flow and vacuum, and neither directly infers mass. Consequently, carburetors are precise, but not accurate fuel metering devices. Carburetors were replaced by more accurate fuel metering methods, such as fuel injection in combination with an air mass flow sensor (MAF).
EGR testing
With OBD II standards, vehicle manufacturers were required to test the exhaust gas recirculation (EGR) valve for functionality during driving. Some manufacturers use the MAP sensor to accomplish this. In these vehicles, they have a MAF sensor for their primary load sensor. The MAP sensor is then used for rationality checks and to test the EGR valve. The way they do this is during a deceleration of the vehicle when there is low absolute pressure in the intake manifold (i.e., a high vacuum present in the intake manifold relative to the outside air) the powertrain control module (PCM) will open the EGR valve and then monitor the MAP sensor's values. If the EGR is functioning properly, the manifold absolute pressure will increase as exhaust gases enter.
Common confusion with boost sensors and gauges
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 100 kPa. This is commonly referred to as gauge pressure. 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 -100 kPa for boost pressure. Thus, a MAP sensor will always read 100 kPa more than a boost sensor measuring the same conditions. A MAP sensor will never display a negative reading because it is measuring absolute pressure, where zero is the total absence of pressure. Vacuum is measured as a negative pressure relative to normal atmospheric pressure. Vacuum-Boost sensors can display negative readings, 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. The suction is caused by throttling in spark ignition engines and is not present in diesel engines. This is often called vacuum pressure when referring to internal combustion engines.
In short, in a standard atmosphere most boost sensors will read one atmosphere less than a MAP sensor reads. At sea level one can convert boost to MAP by adding approximately 100 kPa. One can convert from MAP to boost by subtracting 100 kPa.
A carburetor is a device used by a gasoline internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the Venturi tube in the main metering circuit, though various other components are also used to provide extra fuel or air in specific circumstances.
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline, diesel engines and some hydrogen engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. The exhaust gas displaces atmospheric air and reduces O2 in the combustion chamber. Reducing the amount of oxygen reduces the amount of fuel that can burn in the cylinder thereby reducing peak in-cylinder temperatures. The actual amount of recirculated exhaust gas varies with the engine operating parameters.
Aircraft engine controls provide a means for the pilot to control and monitor the operation of the aircraft's powerplant. This article describes controls used with a basic internal-combustion engine driving a propeller. Some optional or more advanced configurations are described at the end of the article. Jet turbine engines use different operating principles and have their own sets of controls and sensors.
The LA engines are a family of pushrod OHV small-block 90° V-configured gasoline engines built by Chrysler Corporation. They were factory-installed in passenger vehicles, trucks and vans, commercial vehicles, marine and industrial applications from 1964 through 2003. Their combustion chambers are wedge-shaped, rather than polyspherical, as in the predecessor A engine, or hemispherical in the Hemi. LA engines have the same 4.46 in (113 mm) bore spacing as the A engines.
A blowoff valve is a pressure release system present in most petrol turbocharged engines. Blowoff valves are used to reduce pressure in the intake system as the throttle is closed, thus preventing compressor surge.
In automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders. The word manifold comes from the Old English word manigfeald and refers to the multiplying of one (pipe) into many.
Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.
The anti-lag system (ALS) is a method of reducing turbo lag or effective compression used on turbocharged engines to minimize turbo lag on racing or performance cars. It works by delaying the ignition timing and adding extra fuel to balance an inherent loss in combustion efficiency with increased pressure at the charging side of the turbo. This is achieved as an excess amount of fuel/air mixture escapes through the exhaust valves and combusts in the hot exhaust manifold spooling the turbocharger creating higher usable pressure.
The Honda F-Series engine was considered Honda's "big block" SOHC inline four, though lower production DOHC versions of the F-series were built. It features a solid iron or aluminum open deck cast iron sleeved block and aluminum/magnesium cylinder head.
A mass (air) flow sensor (MAF) is a sensor used to determine the mass flow rate of air entering a fuel-injected internal combustion engine.
An engine control unit (ECU), also called an engine control module (ECM), is a device which controls multiple systems of an internal combustion engine in a single unit. Systems commonly controlled by an ECU include the fuel injection and ignition systems.
Carburetor heat is a system used in automobile and piston-powered light aircraft engines to prevent or clear carburetor icing. It consists of a moveable flap which draws hot air into the engine intake. The air is drawn from the heat stove, a metal plate around the exhaust manifold.
MAFless tuning is a method of operating the fuel injection system on a gasoline-powered motor vehicle whereby the mass airflow meter, or MAF, is removed.
A crankcase ventilation system removes unwanted gases from the crankcase of an internal combustion engine. The system usually consists of a tube, a one-way valve and a vacuum source.
A throttle is a mechanism by which fluid flow is managed by constriction or obstruction.
A heated air inlet or warm air intake is a system commonly used on the original air cleaner assemblies of carburetted engines to increase the temperature of the air going into the engine for the purpose of improving the consistency of the air/fuel mixture to reduce engine emissions and fuel usage. This is especially useful during cold or winter climates, when the engine is being started, to help with initial combustion and to bring the engine to optimum operating temperature.
In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.
Trionic T5.5 is an engine management system in the Saab Trionic range. It controls ignition, fuel injection and turbo boost pressure. The system was introduced in the 1993 Saab 9000 2.3 Turbo with B234L and B234R engine.
Manifold injection is a mixture formation system for internal combustion engines with external mixture formation. It is commonly used in engines with spark ignition that use petrol as fuel, such as the Otto engine, and the Wankel engine. In a manifold-injected engine, the fuel is injected into the intake manifold, where it begins forming a combustible air-fuel mixture with the air. As soon as the intake valve opens, the piston starts sucking in the still forming mixture. Usually, this mixture is relatively homogeneous, and, at least in production engines for passenger cars, approximately stoichiometric; this means that there is an even distribution of fuel and air across the combustion chamber, and enough, but not more air present than what is required for the fuel's complete combustion. The injection timing and measuring of the fuel amount can be controlled either mechanically, or electronically. Since the 1970s and 1980s, manifold injection has been replacing carburettors in passenger cars. However, since the late 1990s, car manufacturers have started using petrol direct injection, which caused a decline in manifold injection installation in newly produced cars.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
