Cycling UK Forum

Carburettors have a kick-down mechanism which squirts a jet of neat fuel into the manifold when you stamp suddenly on the throttle for overtaking and the like. Maximum power is not the same as maximum efficiency, so I doubt the optimum fuel/air ratio is going to be the same for maximum power as it is for maximum efficiency.

You’re right, it isn’t at all, and since maximum NOx output occurs at iirc maximum power, due to maximum temperature, there’s another peak to manage too, and a load of unburned fuel and particulate emissions to worry about as well, and preventing engine knock, and preventing intermittent misfiring.

Theoretical perfect combustion in a petrol engine is with a ratio of just under 15:1 by mass, but even in the best-designed engine that is unobtainable in practice, so with a modern engine management system the ratio in use at any moment probably varies from as a low as 9:1 at start and very sudden high demand, to a good deal over 15:1 on undemanding cruising where excess oxygen, or in some cases recirculated exhaust gas, is supplied to reduce NOx output. Most of the time it’s probably running c12:1.

Oh, and the ratios all have to change if the fuel changes significantly, because of different calorific values and burn characteristics.

In my previous, I was trying to find a form of words that conveyed the principle, and were also applicable to all internal combustion engines and to the fuel-air mix of external combustion (steam) engines too (not sure if I succeeded in the latter).

Anyway, this is all a bit more like a car or motorbike forum than a pushbike forum, and I’ll doubtless get in trouble if I keep talking about it.