Exhaust Temperature Range & Efficiency for 48cc 4-Stroke Gas Engines

[blumist]

I wanted to know what exhaust temperature range would i get with a 48cc 4 stroke single cylinder gasoline engine operating at standard parameters.
Also, what variation in engine efficiency would i get by varying exhaust temperatures using water as a coolant?

 

[xxChrisxx]

Are you talking about injecting water into the cylinder or just cooling the exhaust?

Also there is no way to realistically say what an exhaust temerature would be as there are too many variables.
What is the aim of this?

 

[brewnog]

Temperature between 200deg C and 1,300deg C depending on operating conditions.
Minimal mechanical efficiency change as a direct result of water cooling the exhaust gas.

 

[blumist]

Thanks for the replies,

Thats quite a wide range of temperatures. I wanted to run some theoretical stress analysis on the piston head for the power stroke. I'm having a hard time figuring out an effective approach to calculate the pressure variation for this specific engine as a function of crank angle.
I have an idea of my own which implies the use of a stoichiometric mixture of air and gas occupying the clearance volume right at the start of the power stroke. I would then get the total heat released by complete combustion of the fuel which in turn would get me the initial temperature of the mixture. I could then get the pressure using the gas laws. Integrating it for the complete power stroke i might be able to get somewhere. Should i go for it?
Thanks!

 

[brewnog]

You do indeed need a cylinder pressure curve, but exhaust temperature isn't going to let you work back to this. You either need a measured curve, or to be able to make a pretty good estimation for one and your exhaust conditions won't give you this.

You will also need to look at how much heat you're putting into your combustion chamber and make some calculations about heat loss through the cylinder head and chamber walls so that you can incorporate piston temperatures into your model.

 

[xxChrisxx]

Make one up, you can make assumptions from the ideal otto cycle. If you mix this with measured values you can get a fairly accurate representation.

In true Blue Peter style, here's one I made earlier...

This was made for a 1930s Austin 7 engine. It wasn't a running unit so I had to make assumptions about the power stroke, I set the peak force aTDC (13deg after) and from an old 1940s book on Aircraft piston engine design I got the rest of my assumptions. Peak pressure is 75% of ideal pressure.

I did some measurements for the breathing to get pumping losses. etc etc

I've forgotten the exact details as it was a couple of years since I did this.With the inertia forces to calcualte torque, I got a power output to just over 5% error from the manufacturers stated HP. It was only technically valid at this single RPM rating and the further you go from it the worse the error becomes as it assumed a constant BMEP. But it's not a bad reconstruction from essentially no data.