Quote:
Originally Posted by serialk11r
So I used to think the heat capacity ratio was important, but air's heat capacity ratio falls with temperature, and EGR has a low heat capacity ratio too, so water isn't really different. You can think of water like ultra-cold EGR.
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The heat ratio is important. Like with anything, there's pros and cons. With EGR you can open up the throttle more, advance timing, use higher compression ratios, etc., etc., etc. So, just like with water injection, there are efficiency advantages at the same time. But do too much and you hurt efficiency. Or just add something with a worse heat ratio and not adjust things elsewhere and you hurt efficiency.
The heat capacity ratio is an important part of the thermodynamic equation. You can't get better efficiency than the the thermodynamic equation. And if you lower the heat ratio number, guess what, your thermodynamic efficiency also lowers unless you increase the compression ratio at the same time.
Efficiency = 1 - 1/CR^y-1
For an example, a 1.4 heat ratio gives a 10:1 CR a maximum thermodynamic efficiency of about 60%. But 1.3 gives the same 10:1 CR a maximum thermodynamic efficiency of about 50%. And the problem is that the efficiency gains are even less at even higher compression ratios. In reality, with a 1.3 heat ratio you'd need a 21:1CR to match the same efficiency of a 10:1CR with a 1.4 heat ratio working fluid of around 60%.
This is why it makes sense to use air made of 70% nitrogen. Nitrogen has a higher specific heat ratio than CO2 or water.
This is why non-internal combustion engines (external combustion engines) tend to get better efficiencies with fluids with higher heat ratios like hydrogen or helium.