Based on a previous post regarding the exact same topic , I unfortunately cannot elaborate on what I already know and where my gaps in understanding lie because my post would be removed. Instead, I was told to explain that in response to others’ comments.
In: 8
Because of the Carnot Efficiency wich is the theorethical maximum for converting heat to kinetic energy. It’s based on the temperature difference between hot basin (burning fuel) and cold basin (athmosphere).
It’s calculated with 1-(T_cold/T_hot) when you have the temperature in Kelvin. So to reach a theorethical maximum of 50% your fuel has to burn at ~550K. And then you have to subtract all the losses for friction, heat loss in wrong places etc. etc.
In simpler terms: thermodynamics don’t allow converting heat to movement without lots of losses.
My take: An internal combustion engine (ICE) is a complicated piece of engineering with many moving parts. There are multiple loss mechanisms, from the friction between the moving surfaces to such as the “pumping loss” due to both the need to bring in the fuel-air mixture to the cylinder and due to the need to went out the exhaust gasses (compared to the ideal theoretical engine). These losses do not scale nicely to the domain where we want to use our engines (we usually want a relatively low power level compared to the maximum power possible from a given engine size, like drawing some 10–20 horsepower from the well over 100 hp available even in a small car).
But, with modern engines, either due to smaller displacement and a turbocharger or due to use of Atkinson cycle (for reasons I will not dive into here), the question starts to be more like “why it is difficult to exceed 40% by any significant margin”. The move up from 30% to 40% is quite significant improvement over the classic rule of thumb of 30%.
Should we further reduce the need to excessive power reserves (say, a in a petrol-hybrid with a proportionally weak ICE and much more aggressive use of the electric engine on all accelerations), this could be further increased to likely closer to 50%. We do already have such engines in applications where the peak power reserves are not needed over the static power need (somewhat surprisingly, in racing cars, as their engines are consistently operated at or near their peak power). Pushing towards this limit, one needs to be much more careful with the engine design. Ideally, one may want to only load the engine in a narrower range of revolutions, as the engine will operate with a much lesser safety margin in terms of peak temperature inside the cylinders (or potentially just one cylinder, given the low maximum static power need, like when cruising a long uphill on the motorway).
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