Why irreversibility hurts internal combustion engine efficiency so much | Auto Expert John Cadogan

So, the first law of thermodynamics says, essentially, ‘you can’t win’. Like, when you win at a casino, you walk in with $100 and you walk out with $1000. You can’t do that with energy in an isolated system. It’s impossible.

The second law says - and this is also not how it’s taught, but it should be - it says ‘you can’t break even’. So if you walk into that casino with $100, the best you can hope for is to be hopping in a cab at the end of the evening with $75 still in your pocket, in energy terms.

The second law explains the trend to universal randomisation, quantified by a property called entropy, which we’ve talked about, and which is a measurable thing like mass or length. It gives rise to a property called exergy, which is kinda like energy, except it’s really just the ‘available amount of energy’ in a system.

Like, there’s energy in exhaust gas - we know that, right? But the amount that’s available to do work, potentially, depends on a number of factors, like the temperature outside, and the pressure of the air the exhaust is working against, trying to escape.

The available energy is always less than the total energy, because of laws one and two - because you can’t win or even hope to break even. #science

The whole exergy thing was invented by another dead brainiac named Willard Gibbs in 1873. So you can Google ‘Gibbs free energy’ and knock yourself out with that. Literally.

In an engine, if you do a whole second law analysis there’s this concept of ‘total irreversibility’ which is essentially the sum of irreversible processes - including pumping losses and irreversible heat losses and and combustion irreversibility - and this is why I said in that earlier report that it’s better to think about this in terms of the first law, because the second law is far more likely to make your brain bleed.

The total energy minus the exergy equals the energy you lose access to - the stuff with which you can’t ever use to do anything useful. It’s like anti-exergy. Anti-exergy is generated by irreversibility. And that’s the speed limit on efficiency.

All the energy left in the exhaust products - the chemical energy and the heat you throw away, and all the heat otherwise lost to the environment are irreversible losses, if you’re wearing your ‘second law’ (not breaking even, ever) hat.

It’s just gone. And in its place you are left holding a big, fat wad of anti-exergy. Because the universe says you are not allowed to break even.

Just to quantify this, for those of you who are not yet lying in an ambulance speeding towards an emergency department with the neurosurgery resident standing by, the dudes at Oak Ridge National Laboratory in the US found second law combustion irreversibility losses were about 24 per cent in normal driving conditions, on a 1.9-litre turbodiesel test engine.

It’s the biggest single chunk of anti-exergy in the engine. In addition to that, you lose about 12 per cent for irreversible heat loss in the engine, 11 per cent for friction, seven per cent for irreversible volumetric efficiency kinds of losses, and six per cent for pumping losses (all irreversible anti-exergy deals).

I’m trying to explain all this without equations, because as soon as you jump down that rabbit hole you’re in a engineering lecture, and that’s like cognitive Guantanamo - and I just don’t want to do this to you. But I assure you it’s mathematically provable.

That bit I mentioned earlier: About just having to accept some things are true because the scientific process is robust and there’s really no doubt about it - if you’re not getting this it’s simply because you’re still making the new connections upstairs, and you just have to go with it.

To lubricate this process, here’s a quote from Professor David E Foster, from the university of Wisconsin-Madison, who wrote a compelling but neurologically dangerous, report called ‘Pragmatic Efficiency Limits for Internal Combustion Engines’:

“...because we use an unconstrained chemical reaction as part of the energy conversion process approximately 20 to 25 percent of the fuels available energy is destroyed. As long as unrestrained chemical reaction is used in our propulsion systems with current combustion temperature ranges, this loss is unavoidable.”

This also applies to fuel cells - because they’re not closed system or reversible either. And coal-fired power plants.

I get that your gut might not be across the line on this yet, but intellectually I think you need to accept the truth that there’s a speed limit on engine efficiency of about 75 per cent, brought to you by the ‘you can’t break even’ second law. Thanks very much. Would you like some more exergy with that? That’s the challenge.

At least now you know what exergy is - so that’s a plus.