When the engine speed in my petrol car drops below about 650-700 RPM - the normal idle speed - it starts to shake and judder. This could be because say, I didn't give enough gas when pulling away.

Why does it run really badly and shake?

Also, what determines the idle speed in design?

EDIT: I want to know why engines run badly when the idle speed is lower, and how the idle speed of an engine is determined when it is designed, not why it stalls.

  • mechanics.stackexchange.com/a/12754/4152 ... does this answer your question? Sep 7, 2015 at 18:11
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    While I agree the two questions are not exactly the same, what you are describing (running more poorly as the engine speed gets slower) is the classic example of stalling. I'll retract my close vote, but realize what I've written in my other answer should explain everything to you. Sep 7, 2015 at 22:15

2 Answers 2


The engine does not rotate smoothly. The crank shaft is actually constantly speeding up and slowing down. This is most easily seen in a single cylinder engine. The power stroke is the only time the engine is under power. The other three phases are completely unpowered. This means that the engine needs to store enough power in something to get it through the unpowered phases. This is normally done with the flywheel and harmonic balancer. They have a high enough mass that when they are sped up by the power stroke they will keep the engine spinning util the next power stroke comes around.

This action limits the lowest speed of the engine because if enough energy is not stored in the flywheel then the engine has a hard time making it to the next power cycle. The reason there is not enough energy is because to get the engine to spin slower and slower you add less and less fuel. Eventually you reach that limit.


There are a lot of rotating parts in the engine, e.g. shafts, etc. Below idle they enter their critical revolutions, then they start to wobble (Layman's term) and you feel it inside, in some case in the trucks and tractors you can even see it from the outside, especially when they have troubles igniting the engine (then the whole vehicle is almost jumping).

The above can be reinforced by the non-periodic engine ignition, when cylinders are ignited at very low rates and they almost fire one by one.

To answer your second question after the edit:

Idle speed of engine is determined by the characteristics of the engine's largest rotating parts camshaft, crankshaft, etc(such that they avoid critical RPM's) and by cylinder number and arrangement, i.e. straight engine, V - engine, W - engine, Wankel - engine (their ignition sequences are different and there is a lower bound on the rate of firing of the cylinders).

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