Quote:
Originally Posted by FishStretcher
I don't see that 210 as a bogus number. Others achieve it on other outboards and the quasi-adiabatic heating during compression could explain it on a tight 4 stroke.
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Fish is thermodynamically correct! Adiabatic heating could explain those high pressures. For the less technically inclined, the term
adiabatic means that there is no heat loss in the process. Although some heat is actually transferred to the piston, cylinder and head, at high speed the process happens so fast that there is very little time for much heat transfer to occur. Compression ratio is actually the ratio of volume of the cylinder + combustion chamber to the volume of the combustion chamber, not the ratio of pressures before and after compression that I had used as a "quick and dirty" analysis of the numbers posted. Although the pressure ratio is approximately the same as the volume ratio, when the heat generated by an ideal frictionless reversible (isentropic) compression process is not allowed to escape, there will be an additional increase in pressure, similar to what you would get if you simply increased the temperature of a gas in a small constant volume space equal to the same temperature rise you’d get with from an isentropic compression process. This is why diesel engines are able to generate cylinder temperatures that are well above the flash temperature of diesel oil.
What’s interesting as a practical matter is how Suzuki is able to run those compression pressure (and corresponding temperature) levels on 87 octane fuel without getting some serious detonation. They’ve demonstrated some good engineering practice with the chain cam drive and offset driveshaft/extra gear reduction, so it wouldn’t surprise me if they also used oil jets built into the connecting rods for additional piston cooling and an extraordinary amount of cooling water flow in the cylinder heads, so that significant heat is removed from the cylinder during operation. Although these cooling losses hurt thermodynamic efficiency, they might allow you to run cheap low octane fuel. An additional good (but expensive) practice would be to bore and hone the cylinders with a head plate bolted to the block so the cylinders are perfectly round in the assembled condition, creating a better ring seal which would help generate those high compression numbers. (BMW does this on their production engines and it’s a common hot-rod trick done on blueprinted race engines.) The oil and water cooling don’t exist at cranking speeds, so you’d tend to get the adiabatic heating effect during a compression test, while normal operation would not be adiabatic because of the additional piston and cylinder head cooling. This additional oil and water cooling would also improve durability, and that’s something you’d want in an otherwise highly stressed engine that’s generating 140 hp from only 122 cubic inches!
I think the 140 Zuke just uses a simple port-type EFI system like most cars that only requires about 35 psi in the fuel rails. A DFI system requires either an expensive 600 psi fuel pump like the HPDI motors use, or powerful injectors like the E-TEC that create 600 psi at the nozzle using only a 35 psi fuel pump. As Friz and Blue Heron were implying, you can get a lot of cooling out of that DI fuel spray however, which would allow you to run higher than normal compression. Modern turbo and/or high compression (~12:1) engines now feature DI and this would be the next logical step if Suzuki wanted get even more power out of that block, but you’d pay for it with higher hardware costs!