What is limiting reliable HP and how to get past it?
Ok, like the title says I’d like to start a discussion on what limits our horsepower and how to get around it. First I’ll share what I think.
I believe we have most of the answers now. The problems we have with engine reliability stem from three things:
Not enough octane:
The combustion pocket shape makes it highly susceptible to knock, this is partially countered high surface area and the combination of water/oil cooling. Octane becomes more essential at higher RPMs, and at higher compressions. Since our engine runs at high compression and high RPMs it needs high octane out of the box. Add boost and it gets worse. This is why we have seen decent gains NA with e-85.
Solutions: Race Gas, E-85, Water injection.
Not enough cooling:
As stated above the combustion pocket is cooled on both sides. Operation in boost adds a little bit to the heat load to everything. Backpressure keeps the pressure in the combustion chamber from dropping as far during the exhaust phase, which keeps temperature from dropping as far, which raises the average temperature the rotor is exposed to, and increases the temperature parts of the housing and irons are exposed to. This also increases the average temperature the leading spark plug is exposed to.
Putting an intercooler in front of the radiator makes the cooling problem worse.
Solutions: Water injection, Additional Radiators, Oil Coolers, Colder spark plugs.
Improper Timing:
Ok, this one is going to get me flamed, but I think timing is a huge issue. Because of the compression difference we will inherently run less timing then other rotary engines. The cooling and octane issues are amplified with bad timing. Detonation leads to more heat transferred to all surfaces including the spark plugs, eventually that heat transfer causes power to be lost, and after that it leads to engine damage either from the detonation or from preignition. Even if the timing doesn’t lead to immediate engine damage, the increased heat transfer to the side irons will cook the side seal springs. But on the other side of the coin running too retarded will cause high EGTs and damage side seals, or at least that’s the current belief.
Split is a bit of black magic. You can get higher power with smaller split. In theory you could even run a negative split in boost for power gains, but if you ever lean out unintentionally engine destruction will quickly follow.
I’m currently working on different ignition timings and testing the water. Still waiting on aspects of my build to support further experimentation.
Solution: Push a fairly conservative timing, looking to limit EGT instead of pushing power. Run a higher split at higher boosts. Possibly disabling the trailing plug at high rpm (since blanking is not an option for idle). Push boost instead of timing. This is not a comprehensive solution, it will evolve as more people are successful.
Ok, like the title says I’d like to start a discussion on what limits our horsepower and how to get around it. First I’ll share what I think.
I believe we have most of the answers now. The problems we have with engine reliability stem from three things:
Not enough octane:
The combustion pocket shape makes it highly susceptible to knock, this is partially countered high surface area and the combination of water/oil cooling. Octane becomes more essential at higher RPMs, and at higher compressions. Since our engine runs at high compression and high RPMs it needs high octane out of the box. Add boost and it gets worse. This is why we have seen decent gains NA with e-85.
Solutions: Race Gas, E-85, Water injection.
Not enough cooling:
As stated above the combustion pocket is cooled on both sides. Operation in boost adds a little bit to the heat load to everything. Backpressure keeps the pressure in the combustion chamber from dropping as far during the exhaust phase, which keeps temperature from dropping as far, which raises the average temperature the rotor is exposed to, and increases the temperature parts of the housing and irons are exposed to. This also increases the average temperature the leading spark plug is exposed to.
Putting an intercooler in front of the radiator makes the cooling problem worse.
Solutions: Water injection, Additional Radiators, Oil Coolers, Colder spark plugs.
Improper Timing:
Ok, this one is going to get me flamed, but I think timing is a huge issue. Because of the compression difference we will inherently run less timing then other rotary engines. The cooling and octane issues are amplified with bad timing. Detonation leads to more heat transferred to all surfaces including the spark plugs, eventually that heat transfer causes power to be lost, and after that it leads to engine damage either from the detonation or from preignition. Even if the timing doesn’t lead to immediate engine damage, the increased heat transfer to the side irons will cook the side seal springs. But on the other side of the coin running too retarded will cause high EGTs and damage side seals, or at least that’s the current belief.
Split is a bit of black magic. You can get higher power with smaller split. In theory you could even run a negative split in boost for power gains, but if you ever lean out unintentionally engine destruction will quickly follow.
I’m currently working on different ignition timings and testing the water. Still waiting on aspects of my build to support further experimentation.
Solution: Push a fairly conservative timing, looking to limit EGT instead of pushing power. Run a higher split at higher boosts. Possibly disabling the trailing plug at high rpm (since blanking is not an option for idle). Push boost instead of timing. This is not a comprehensive solution, it will evolve as more people are successful.
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