Hows a car engine work in regard to fuel efficiency?

Question

i watched a TV show yesterday where a driving instructor was teaching how to get the best fuel efficiency out of a car. he stated that if your foot isn't on the gas, then the injectors will be switched off and no fuel will be used? but if a car is idling it is still burning fuel right? so does the engine know the difference between idling when the car isn't moving and when the car is in gear but moving under momentum and not being driven by its engine?

Answer 1

Simply, the car engine does not shut completely off at idle and of cource you are using something of fuel/air to maintain an idling/running engine.  However the computer is put into a minimum-to-operate mode at coast or idle.  This obviously will require less fuel than if you are demanding additional power to push your vehicle with anything more than min. idle or coast mode.  Some computers will actually shut off the injectors when you are coasting down (but not at idle) as this does not demand any push from your engine and actually the car is pushing the engine into rotation to some degree!

Hope this helps....

Answer 2

Yes, all car engines are basically air pumps and by measuring the air into and out of the engine the load can be determined.

So, yes, the modern car engines electronics know when your coasting and can indeed cut off the fuel injectors for a short period of time.

Answer 3

It doesnt switch the injectors off it just cuts the amount of fuel delivered to the injectors.

Answer 4

the car this person was probably referring to is either a hybrid or a car with MDS (multiple displacement system). in those cases, the computer tells the injectors to stop working when the accelerator is not being pressed.

for all other cars, fuel is constantly being pumped into the engine. just at a slower rate.

an engine works sort of like an air pump. fuel is sucked into the combustion chamber when the piston moves down creating a vacuum. the fuel is mixed with air and is compressed then ignited by the spark plug which forces the piston back down for power. there are 4 cycles to a car engine...

1)intake of air and fuel

2)compression and ignition of the air/fuel mix

3)the "power" stroke when the ignition of the mix forces the piston down to create torque

4)exhaust is expelled when the piston moves back up.

stepping on the gas peddle introduces more fuel and air into the system causing it to move faster. letting off the peddle does the opposite. even with no pressure on the peddle, the engine still continues the process, just at a slower rate. If your car has a tachomoter you can watch the RPM (revolutions per minute) rise and fall when you hit the gas.

fuel is constantly being pushed into the motor by the fuel pump. on more modern computer controlled cars, there are sensors that tell the engine how to manage it's fuel consumption better. these are things like the Throttle Positioning Sensor or TPS which gives a more accurate depiction of how much the gas peddle is being pressed than just by a cable (the more traditional way of doing things).

all in all, as long as the ignition is turned on, the car is consuming fuel. idling uses the least fuel. coasting is next best because if the car is in gear (even if the gas peddle is not being pressed), the transmission creates drag on the motor causing it to burn more fuel.

hope this is not too confusing.

Answer 5

The engine without any fuel supply will stall,the amount can be massively reduced but it still requires some to carry out combustion,without combustion the engine breaking effect is quite an effective brake  unless the exhaust valves are held open and there is no piston / valve contact ...