Associate
- Joined
- 30 Dec 2002
- Posts
- 738
A modern fuel injected car will work something like this. If we assume we are dealing with an indirect fuel injection system then somewhere upstream of the intake valve(s) there will be a number of fuel injectors, if we assume we are dealing with a multi-point petrol injection system there will be 1 injector per cylinder.
The injector is quite a simple piece of equipment, if it isn’t receiving an electronic signal it remains closed, if it does receive an electronic pulse from the ECU it opens and sprays pressurised fuel, the rate of fuel flow is constant so the amount of fuel injected is directly proportional to the length of time the injector stays open, hence the ECU can control the amount of fuel injected into the cylinder by altering the length of the electronic pulse.
Using various sensors, including the MAP / MAF sensors (which allow the ECU to calculate the amount of air being drawn into the cylinders) the oxygen sensor (which allows the ECU to calculated how much of that air is oxygen) the ECU can calculate the amount of fuel needed to create the ideal fuel-air mixture.
If the throttle is closed the ECU will realise that there is no need for the engine to create any power, thus the fuel supply is cut off, this means the injectors receive no electronic pulse and therefore stay closed. Some air will be drawn into the engine by the vacuum created on the intake stroke, and the spark plug will continue to spark burning any fuel that might accidentally end up in the cylinder. As soon as the throttle is opened again the ECU begins sending the injectors electronic pulses, therefore fuel enters the cylinders and combustion begins again, this happens almost instantly and isn’t noticeable to the driver.
If the clutch is engaged the engine will be driven by the cars wheels and the engine and car speed will slowly drop because of friction etc. If the clutch is disengaged the engine will slow down much quicker because the only turning force will come from the momentum of the flywheel and other engine components, as the flywheel weighs much less than the entire car frictional forces will slow the engine down much quicker.
When the engine falls below a certain rpm with the throttle closed, the ECU will switch into idle mode, to stop the engine from stalling due to a lack of oxygen and fuel the ECU will start sending pulses to the fuel injectors and also open a throttle bypass valve which allows air into the engine even though the throttle is closed.
As you can probably tell slowing down by closing the throttle and keeping the clutch engaged is much more fuel efficient then disengaging the clutch and allowing the engine speed to fall to idle while the car coasts, it is also safer to keep the clutch engaged as it gives you more control of the car while slowing down. It is important to remember to change down gears in good time while engine braking as if the engine rpm becomes too low the ECU will start injecting fuel to prevent the engine stalling, also as no fuel is being used higher engine speeds do not impact fuel economy as they would do under acceleration.
The injector is quite a simple piece of equipment, if it isn’t receiving an electronic signal it remains closed, if it does receive an electronic pulse from the ECU it opens and sprays pressurised fuel, the rate of fuel flow is constant so the amount of fuel injected is directly proportional to the length of time the injector stays open, hence the ECU can control the amount of fuel injected into the cylinder by altering the length of the electronic pulse.
Using various sensors, including the MAP / MAF sensors (which allow the ECU to calculate the amount of air being drawn into the cylinders) the oxygen sensor (which allows the ECU to calculated how much of that air is oxygen) the ECU can calculate the amount of fuel needed to create the ideal fuel-air mixture.
If the throttle is closed the ECU will realise that there is no need for the engine to create any power, thus the fuel supply is cut off, this means the injectors receive no electronic pulse and therefore stay closed. Some air will be drawn into the engine by the vacuum created on the intake stroke, and the spark plug will continue to spark burning any fuel that might accidentally end up in the cylinder. As soon as the throttle is opened again the ECU begins sending the injectors electronic pulses, therefore fuel enters the cylinders and combustion begins again, this happens almost instantly and isn’t noticeable to the driver.
If the clutch is engaged the engine will be driven by the cars wheels and the engine and car speed will slowly drop because of friction etc. If the clutch is disengaged the engine will slow down much quicker because the only turning force will come from the momentum of the flywheel and other engine components, as the flywheel weighs much less than the entire car frictional forces will slow the engine down much quicker.
When the engine falls below a certain rpm with the throttle closed, the ECU will switch into idle mode, to stop the engine from stalling due to a lack of oxygen and fuel the ECU will start sending pulses to the fuel injectors and also open a throttle bypass valve which allows air into the engine even though the throttle is closed.
As you can probably tell slowing down by closing the throttle and keeping the clutch engaged is much more fuel efficient then disengaging the clutch and allowing the engine speed to fall to idle while the car coasts, it is also safer to keep the clutch engaged as it gives you more control of the car while slowing down. It is important to remember to change down gears in good time while engine braking as if the engine rpm becomes too low the ECU will start injecting fuel to prevent the engine stalling, also as no fuel is being used higher engine speeds do not impact fuel economy as they would do under acceleration.
5* post.