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  1. #23
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    Apr 2007
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    Default Re: British Hydrogen

    Quote Originally Posted by Mega View Post

    What a great find Mike. I love exotic and unusual engine concepts.

    I put this one into the bucket called "complete expansion schemes". By habit and tradition we say "compression ratio", but it's just as correct to say "expansion ratio".
    The theoretical appeal of achieving complete expansion has lured engineers since the early days of steam locomotives. A lot of clever work was done for steam engines more than a hundred years ago, trying to get complete expansion of the steam before it left the engine. Some were like this concept, with a second or third downstream cylinder of a different size and shape. The downstream cylinder accepts the exhaust of the upstream cylinder, expanding it further and extracting the last bit of energy from the fuel.

    The same fundamental principal limits all these schemes and prevents complete expansion. Friction. There's significant friction in piston seals and valve mechanisms. The extra friction in the complete expansion machinery uses up all the extra energy you extract by further expansion. You end up with no gain. Take it too far and you might end up with efficiency losses. Although in theory you can gain efficiency, in the real world you hit a limit.

    Improvements in materials and design are creeping ahead and slowly improving the practical limit on complete expansion. If you want to know the current limit, just look at the compression ratio of the newest and best heavy diesel engines. I just looked at the new Cummins X15 engines and see they run about 19:1. It's been many years since I looked and I'm delighted by 19:1. Back in the 1990s I would have used 16:1 as a rule of thumb. I suspect all of this improvement traces back to better materials for piston rings and valves that have lower friction and can tolerate higher temps.

    So far in the real world, it's always proved better to have a single cylinder with a higher compression ratio instead of having extra downstream cylinders each with lower compression ratios.

    It's notably different for turbines. The big steam turbines in power plants worldwide have separate high pressure and low pressure turbines. Just like the two cylinders in your video, the low pressure turbine downstream accepts the exhaust of the upstream high pressure turbine, and the two have different geometries.

    The turbocharger on your car is essentially a separate downstream machine to further expand the exhaust gases. Instead of extracting energy from expansion and putting it directly to the drive shaft, it extracts energy and puts it to it's compressor end, where it raises the expansion ratio of the engine system overall.

    The other big bucket for unusual engine designs I call "super hot combustion schemes". Carnot's theorem tells us the efficiency limit of an internal combustion engine is controlled by the temperature difference between the heat source(combustion temp) and the heat sink (the world). So engineers are forever coming up with engines that can run at absurdly high internal temps to get the overall efficiency better. Those don't work in the real world either, again because of basic materials. But just like compression ratios, internal engine temps have moved slowly higher over the years.
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    Last edited by thriftyandboringinohio; 07-10-20 at 11:04 PM.

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