Quote:
Originally Posted by Bobux
Can I ask for an explanation here? I am not an expert in thermodinamics, but I came to a different conclusion. I thought of it like this:
Example 1:
Assume I had an engine working under athmospheric pressure, CR of 10:1. When the piston reaches it's top position I have 10atm pressure. Burning all air in the chamber the pressure will rise to 40atm because the air will try to expand 4:1. This way extra 30atm will push the piston down and generate torque.
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Hi Bobux,
There seems to be a misunderstanding of the meaning of expansion ratio. A real Diesel engine doesn't work exactly like the theoretical "ideal" cycle, but it isn't any better.
In an ideal Diesel cycle the air is compressed, as you say, 10:1. The pressure goes to 10 atm (and a little more due to compression heating) V1 to V2. About TDC the injector adds fuel, which burns and releases heat at exactly the rate that the volume is increasing, so the pressure stays about 10 atm. The injector adds fuel from b to c. Then as the volume continues to expand the pressure and temperature fall until the piston reaches BDC, point d. The expansion from when the fuel stopped burning at c until BDC at d is the expansion ratio, V3 to V1. At low power the b-c distance will be small and the expansion ratio will be almost 10:1. At full power b-c will be most of the 90 degrees of the power stroke and the expansion ratio will be small. As b-c gets longer and the expansion ratio gets smaller the pressure in the cylinder when the exhaust valve opens is higher. The pressure and temperature in the exhaust when the valve opens is lost, wasted power.
If you add a turbine to the exhaust you can recover that power by expanding the pressure and temperature the rest of the way down to 1 atm. You can use the exhaust power recovered by the turbine to spin a supercharger or compressor and pre-compress the intake air. Adding more air to the intake allows more fuel to be burnt increasing the power output, but ignoring that, the higher compression pressure produces a higher compression temperature. Efficiency can be computed as the high temperature minus the ejection temperature all divided by the high temperature. Including a turbine in the exhaust so the final ejection temperature is the same for different operating conditions, you can see that the supercharger acts just like increasing the CR and increases efficiency. The temperature rise due to compressing a gas is the initial temperature multiplied by the compression ratio raised to the power of g-1, where g stands for the ratio of specific heats for the gas. g = 1.4 for air. So if the incoming air is about 80 F (300 K) and the CR is 10, the compression temperature is 753 K. If the equivalent CR is raised to 20:1, then the compression temperature goes to 994 K If the ejection temperature is 373K (212 F) for both cases, the efficiency for CR = 10:1 is about 50%, and for CR = 20:1 about 60%
Adding a turbocharger is expensive and complicated, but if you boost intake pressure by 1 atm you'd improve fuel economy by 20%
-mort