[bwilson4web]

Originally Posted by jamesqf

Try this one: Switched Reluctance Drives

Thanks! That works better.

Originally Posted by jamesqf

So your exhaust pipe between the exhaust valves and the turbo becomes a pressure vessel. What does the increased back pressure do to the engine?

It extends the engine working gas expansion ratio from 13 to 1 to closer to 14/15 to 1. The turbine has become another part of the engine cycle connected by the exhaust-pipe pressure vessel. In fact, the turbine replaces the muffler, reducing back pressure from that source.

My expectation is:

• engine off hybrid mode - no effect
• normal cruise mode - use of water injection to improve mass flow and kinetic energy to turbine proportional to available engine gas flow
• high power, acceleration and climbing - water injection off as the turbine spins on just the exhaust gas

So let's go back to what we know of this vehicle, the thermodynamic model:

In normal cruise modes, 30-65 mph, the engine power needed is 6.7-29.5 kW. But the gas flow is too low to fully load the turbine unless we use water injection to increase the mass-flow rate.

At peak power, accelerating, climbing or ~100 mph, the 83 kW gas flow is high enough to load the turbine but exceeds the ability of the traction battery to absorb the 27.5-47.6 kW potentially generated. So the power draw is limited to the generator limit, the turbine is unloaded and exhaust pressure reduced.

Near as I can tell, the turbine exhaust ratio is roughly 2:1(*). So instead of 1 bar back pressure, a completely perfect, no loss of energy since the crank case is at 1 bar to push out the exhaust gas. This means the piston now has to do more work, exactly 2 bar cylinder pressure minus 1 bar crank case resistance . . . 1 bar extra pressure on the piston during the exhaust stroke. So let's do the math:

• 75 mm - bore -> 0.0044 square meters
• 101.3 kPa - 2:1 bar pressure difference -> 447.5 newtons
• 84.7 mm - stroke -> 0.0847 meter
• Joules to expel gas with 2:1 cylinder-to-case pressure -> 37.9 J
• Max rpm, 4,500 rpm -> 75 revs/sec
• 1/4th rpm is exhaust stroke -> 243.7 W at full power, fully loaded 2:1 turbine, overhead on engine

So to get potentially 27.9-47.6 kW at full power, we've increased the engine load by 243.7 W. Of course we're going to limit the turbo-alternator to 20,000 W and in real life, there where will be other losses. At lower speeds, the water injection will due to lower rpm, ~2,250 rpm, cost ~122 W to generate 2,220 to 3,840 W. I can live with these trade-offs.

The boundary of the engine has expanded as it now includes the turbo-alternator. There will be an effect on the piston engine but the engine boundary is larger than it was before. So we are gaining mechanical power that in past was lost in the excessive pressure drop when the valves opened. The turbo-alternator has become a less-bad muffler.

Bob Wilson

* - I am not sure what the turbo pressure ratio will be for any given power output. The turbo charger vendors are not terribly forthcoming with the turbine maps but one can easily choose whatever ratio one thinks is practical and use it as a multiplier to calculate the back pressure impact.