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ICE Engines are designed to burn the fuel and air mixtures at high pressures to increase the combustion efficiency. But hydrogen combustion efficiency is already high at 1 atm and at constant Volume combustion, peak pressure may reach around 8 atm. So why compressing the hydrogen and air mixture in Hydrogen ICE engines? If we could avoid compression of fuel and air, we can design much simpler and cheaper Hydrogen engines.

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    $\begingroup$ Using a compression ratio of between 8 or 12 to 1 (gas, but diesel is 18 or more to 1) is done to improve the power output more that combustion efficiency. $\endgroup$ – Solar Mike Nov 5 '18 at 19:06
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SolarMike's comment basically answers the question, but let me flesh it out a little.

Let's take a typical engine, say it has a compression ratio of 12:1, and weighs 500 pounds, and has a displacement of 4 liters. Now let's reduce the compression ratio to only 2:1. To get the same amount of power output, the displacement now how to be 24 liters. (can you see why? burning one-sixth as many fuel molecules per liter, so need six times more liters). So now the cylinders and pistons have to be about six times bigger as well, so now the car/truck has to be a lot bigger overall to fit this giant engine in there. Also those bigger cylinders are going to weigh a lot more, potentially as much as the entire car/truck, which hurts the performance of the vehicle. Overall it just doesn't make any sense to use this kind of engine. The power density is too low.

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  • $\begingroup$ It why car engines are small compared to locomotive engines. Locomotives have to haul are heavy load so they require a lot of power, resulting in a big engine. Cars however, haul much smaller loads & thus need engines with less power. A high compression ratio is nice, but power is what gets the job done. $\endgroup$ – Fred Nov 6 '18 at 13:10
  • $\begingroup$ Thank you for replies. Compare jet engine and pulse jet engine. It is well known that pulse jets are inefficient and one of the main reason is due to their low compression ratios. As per my understanding now, fuel & air mixtures are compressed for both reasons 1. increase power density 2. combustion efficiency. $\endgroup$ – SRD Nov 6 '18 at 23:39

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