Let say I directly connect a small engine to my bicycle, and that I'm superman who can overcome any force. Then I start cycling (using my legs...) - the engine starts to rotate, even though I haven't "started it" using the switch.

Does it mean that if I press the pedal (or whatever) of the engine, it will apply force? Either on a diesel or a gasoline engine.

Another question: Lets say I'm driving down a hill in a relatively high speed in a car, and I'm not pressing the gas pedal at all. Does the engine burn fuel at all, in both diesel and gas? If I (while in gear) turn off the engine, will something actually happen? If I would press the pedal after turning it off (while it still rotates) - will it apply force? (in both diesel and gas).

If, while driving, I disconnected the idler (i.e the engine won't apply any force if I'm not pressing the pedal at any RPM and pressing the clutch will take it to zero RPM) and press the brake until I get to a complete stop, will the engine "stall"? If I'm doing it in a hill facing down, after releasing the brakes I'd accelerate and the engine would gain RPMs, will it be able to continue applying force?

Finally: What's the difference between a "running engine" and one thats not running, assuming both are rotating?

I'm sorry if I'm asking too much so if you would at least explain the principle I'd be very happy. Thank you!

3 Answers 3


All EFI engines shut off the fuel injectors when the accelerator is released above a certain RPM or speed (varies by manufacturer). It's easier to detect on newer model cars with a digital instant fuel economy gauge where when cruising at 55 MPH and the accelerator pedal is released, the economy readout will shoot from 30 MPG to the highest number or the readout goes blank.

On an automatic transmission when the gear selector is placed in neutral at highway speed, the engine computer will allow the fuel injectors to fire again and actually raises the RPMs in anticipation of it being thrown into gear again. On a manual transmission when pressing in the clutch or placing gear selector in neutral, the engine RPMs will fall to idle where the engine computer will fire the injectors to maintain idle speed.

If you turn off the engine (with the key) while moving, the car will still keep moving, but you won't accelerate or maintain speed. If the engine is off, pressing the accelerator does nothing.

  • What is "EFI"? Does every engine have an ECU and "idle"? How about small "low russian" engines? Will they operate even after completely stalling, and then moved by an external force?
    – Mark Segal
    Jul 11, 2013 at 19:30
  • EFI = Electronic Fuel Injection. Only EFI engines have ECUs. I'm not sure what you mean by "idle" because all engines will idle. Not sure about "low russian" engines either. The engine will not operate after stalling until you start the engine again. You can start it by external force if you leave the clutch engaged, put it in 3rd gear, turn key to "on", and push/pull the car. This is called "push starting".
    – Alex
    Jul 11, 2013 at 19:38

I will explain it for an EFI engine with a manual transmission because it might not be as straight forward with a carbureted engine or automatic.

For ease of explaining this I will consider the engine as "on" whenever the ignition switch is set to the "on" state. This means that the ECU is on and is monitoring the engine state and controlling it's behavior.

For the bicycle example with an EFI engine if you had your "ignition" to on while you started cycling then the ecu would start to inject fuel and cause spark so yes it would apply force and use fuel. If the ignition were off then the ecu would be off and you wouldn't get any fuel or spark so no you won't get any force if you apply the accelerator.

Going downhill not pressing the accelerator with the ignition on will use significantly less fuel (not sure if the ecu uses zero fuel though) and the engine will still be sparking for sure. If you were to turn the ignition to accessory while coasting downhill the ecu will shut off and you will stop injecting fuel and stop spark as well so the accelerator would no longer do anything.

If you have the clutch in and you stop the engine will stop. If you then release the brakes and roll down a hill then one of two things will happen.

1) if your ecu is on (ingition on) then the engine will start to use fuel and spark. (It will be really rough until it gets to ~700rpm) 2) if your ecu is off then the engine will rotate but no fuel or spark so therefore no acceleration.

Finally the difference between a running engine and not if they are both spinning :

A running engine is one which can continue to run from the energy it is producing without an external force. Modern engines require fuel air and spark to satisfy this requirement. eg. ECU on and rotating

An engine is off if when all external forces are removed then engine would stop. eg. the ecu is off so the engine isn't producing any of its own power and is only spinning because it is connected to wheels that are being accelerated by gravity.

Hope this clarifies it somewhat....


any engine that is actually running meaning the pistons are spinning under power uses some fuel to even Idle at a stop. Diesels use compression to make the spark in the piston where gasoline uses spark plugs to burn the fuel. The exact amount of fuel entering the engine depends on the specific type of engine. The small lawn mower size engine uses what we called a down draft system based of the air passing through the carburetor nozzles sucking fuel in its path metered by jet valves. Most diesel motors use fuel injection and that requires a pump to push it along. Hoped this helped a bit.

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