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cajunfj40 · 03-17-2017, 12:49 PM

Hello DieselJan,

Huh, this is interesting. I did some rough calcs using a Chevy Duramax Diesel engine at 4,000RPM (a bit higher than recommended, but it can get there) because I could find the relevant numbers. 4.06" bore by 3.9" stroke, 850 gram piston. After converting everything to SI units, I got a peak acceleration force on the piston of about 5kN when it is slowing down from max speed at half-stroke to 0 speed at BDC. With a peak combustion pressure of ~2500PSI, the force exerted on the piston is ~143kN, or about 30x that of the accelerating piston. At least that means that if you have the right spring for peak cylinder pressure it won't move much due to piston acceleration forces.

Trouble is, that high a force will require a spring that won't fit the available space. Converted back to US units, that's a bit over 16 tons of force at peak cylinder pressure. Think needing ~4 main suspension springs from a fully-loaded tractor-trailer rig (80 tons).

You could use a gas spring, but to do that you'll essentially end up with two pistons within the same cylinder, one a "free" piston in contact with the combustion gases, and the other trapping some 2500psi+ gas between it and the combustion piston, with some means to keep them from getting further apart than the height needed for proper compression ratio. Unfortunately, that doubles your piston ring friction and you have to figure out how to keep that gas pressure topped up. Packing the gas spring into the con-rod/piston package would mean higher pressures and more elaborate sealing methods, which may add less friction than a second set of piston rings, but it still is quite an addition.

I don't think you'll get sufficient efficiency gains to offset the additional parasitic loads from the added friction, rotating mass, etc. that your re-design adds. Sorry.

I know you didn't really want a different way to get the same effect - just a critique of your idea - but there are some other things out there that may interest you, so go ahead and ignore the rest of this if you want.

A simpler way of getting more leverage at peak cylinder pressure/TDC is offsetting the piston bore relative to the crankshaft, or offsetting the wrist pin in the piston. Basically, when the piston is at TDC, the crankshaft is rotated a bit beyond the point where the crank arm would be vertical. When peak cylinder pressure occurs, it will be working on a lever arm right away, rather than pushing straight down the vertical stack of piston/wrist pin/conrod/crank bearing/main bearing/main bearing cap, and trying to squeeze the oil films between those parts.

You can also play with combustion pressure profiles by changing how the mixture reacts. There's some really neat work being done out there.

You mention that you are an electronic engineer - check into "Reactivity Controlled Compression Ignition" or similar (U of Wisconsin has some good stuff here: RCCI (Reactivity Controlled Compression Ignition) Engine - Wisconsin Engine Research Consultants - Engine Research Center) where they use multiple injections of fuel of varying or modified octane/cetane rating to control the reactive species mix in the combustion chamber, with a lot of control over peak temperature, peak pressure, etc. They have achieved brake mean thermal efficiency up to 57% with cetane enhancer modified gasoline (example of their work here: High Efficiency, Low Emissions RCCI Combustion by Use of a Fuel Additive). They have an early version ("only" about 47%-52% brake mean thermal efficiency depending on fuels used) that has both port (octane-rated fuel, or "low reactivity") and direct (cetane-rated fuel, or "high reactivity") injection compression ignition in a high-compression engine. This may be in your area of expertise - figure out how to graft the port fuel injection system from a flex-fuel spark ignition engine onto a modern common-rail direct injection diesel engine, and get the two ECU's to talk to each other or be controlled by a third computer, in order to get the RCCI results. If you can come up with the way to get that sort of thing to work, you could open up some really interesting aftermarket modifications that don't require heavy engine wrenching - just appropriate junkyard scrounging and code work, plus an auxiliary fuel system.

03-17-2017, 12:49 PM	#31 (permalink)
cajunfj40 Lurking Eco-wall-o-texter Join Date: Dec 2015 Location: MPLS, MN area Posts: 128 Thanks: 0 Thanked 65 Times in 45 Posts	Hello DieselJan, Huh, this is interesting. I did some rough calcs using a Chevy Duramax Diesel engine at 4,000RPM (a bit higher than recommended, but it can get there) because I could find the relevant numbers. 4.06" bore by 3.9" stroke, 850 gram piston. After converting everything to SI units, I got a peak acceleration force on the piston of about 5kN when it is slowing down from max speed at half-stroke to 0 speed at BDC. With a peak combustion pressure of ~2500PSI, the force exerted on the piston is ~143kN, or about 30x that of the accelerating piston. At least that means that if you have the right spring for peak cylinder pressure it won't move much due to piston acceleration forces. Trouble is, that high a force will require a spring that won't fit the available space. Converted back to US units, that's a bit over 16 tons of force at peak cylinder pressure. Think needing ~4 main suspension springs from a fully-loaded tractor-trailer rig (80 tons). You could use a gas spring, but to do that you'll essentially end up with two pistons within the same cylinder, one a "free" piston in contact with the combustion gases, and the other trapping some 2500psi+ gas between it and the combustion piston, with some means to keep them from getting further apart than the height needed for proper compression ratio. Unfortunately, that doubles your piston ring friction and you have to figure out how to keep that gas pressure topped up. Packing the gas spring into the con-rod/piston package would mean higher pressures and more elaborate sealing methods, which may add less friction than a second set of piston rings, but it still is quite an addition. I don't think you'll get sufficient efficiency gains to offset the additional parasitic loads from the added friction, rotating mass, etc. that your re-design adds. Sorry. I know you didn't really want a different way to get the same effect - just a critique of your idea - but there are some other things out there that may interest you, so go ahead and ignore the rest of this if you want. A simpler way of getting more leverage at peak cylinder pressure/TDC is offsetting the piston bore relative to the crankshaft, or offsetting the wrist pin in the piston. Basically, when the piston is at TDC, the crankshaft is rotated a bit beyond the point where the crank arm would be vertical. When peak cylinder pressure occurs, it will be working on a lever arm right away, rather than pushing straight down the vertical stack of piston/wrist pin/conrod/crank bearing/main bearing/main bearing cap, and trying to squeeze the oil films between those parts. You can also play with combustion pressure profiles by changing how the mixture reacts. There's some really neat work being done out there. You mention that you are an electronic engineer - check into "Reactivity Controlled Compression Ignition" or similar (U of Wisconsin has some good stuff here: RCCI (Reactivity Controlled Compression Ignition) Engine - Wisconsin Engine Research Consultants - Engine Research Center) where they use multiple injections of fuel of varying or modified octane/cetane rating to control the reactive species mix in the combustion chamber, with a lot of control over peak temperature, peak pressure, etc. They have achieved brake mean thermal efficiency up to 57% with cetane enhancer modified gasoline (example of their work here: High Efficiency, Low Emissions RCCI Combustion by Use of a Fuel Additive). They have an early version ("only" about 47%-52% brake mean thermal efficiency depending on fuels used) that has both port (octane-rated fuel, or "low reactivity") and direct (cetane-rated fuel, or "high reactivity") injection compression ignition in a high-compression engine. This may be in your area of expertise - figure out how to graft the port fuel injection system from a flex-fuel spark ignition engine onto a modern common-rail direct injection diesel engine, and get the two ECU's to talk to each other or be controlled by a third computer, in order to get the RCCI results. If you can come up with the way to get that sort of thing to work, you could open up some really interesting aftermarket modifications that don't require heavy engine wrenching - just appropriate junkyard scrounging and code work, plus an auxiliary fuel system.