I have been aware of some combined cycle powerplants for quite a while, and I have not yet heard of anyone attempting to treat the exhaust system of an internal combustion engine like as if it were a steam engine which have a dramatic power increase when the exhaust is ejected into a good vacuum environment. a substantial portion of the engines pumping loss is nearly eliminated by reducing the exhaust backpressure. I am thinking that this high quality exhaust heat of the typical IC engine can be utilised to generate the very vacuum environment that it exhausts into, thus increasing its efficiency. And completely eliminating the mechanical complexity of using the steam to spin up a turbocompressor or to connect directly to the output shaft. Any thoughts or experiences?

You’d have to carefully research the engine you were doing this do — some engines are designed to utilize the back pressure to create a swirl in the cylinder to mix the incoming fuel/air. Removing the back pressure would make the engine run worse, not better. But why go to such a great effort to just provide boost? You can get turbochargers in the junkyard that will provide all the boost the engine can handle – and you’ll still have plenty of heat in the exhaust to make steam.

What else would I use the heat for other than reapplying it to the engine/drivetrain somehow to increase fuel economy? The point of the “Turbo” is to create slightly more backpressure in the exhaust to allow the engine to breathe easier on the intake by compressing even more air into it…it kinda fights itself when you analyse it. The percentage of boost afforded from the exhaust directly is small compared to if the turbo hot section was blown with steam generated from the waste heat instead. the amount of power attainable from blowing the turbo with steam would be something on the order of minimum four times what is usual for the typical spark ignited engine fuels. I would prefer to blow the turbo with steam for a Diesel engine instead because the issue of too much compression in a mixture is absent. at this point, with a steam unit on the engine exhaust it would be possible to have a bigger turbo than normal, or even two units with one slightly larger than the other one in a two stage configuration.

The only other option I can see for using waste exhaust heat for steam would be to use a high horsepower cogbelt attached to the front of the engine with a geared turbine of similar proportions to the hot section of a truck turbo driving the cogbelt, thus adding its horsepower to the whole mix. It is this configuration that is called an augmented blowdown turbine. A similar principle to this was once tried with the largest radial aircraft engine ever flown, the venerable Pratt& Whitney R-4360!! It is a 4360 cubic inch 28 cylinder radial made of 4 rows of 7 cylinders stacked together. The pistons on these are about 7 inches in diameter!! She topped out at 3500 REAL horsepower burning purple avgas (115-145 octane)! She was originally two speed geared supercharged, turbo’d, AND blown down with 4 exhaust turbines directly connected to the primary gearbox at the rear of the engine. the statistics for the exhaust augmented turbines alone was something on the order of 100 hp each, times four!! The exhaust augmentor blowdown turbines were the first to be taken off the engine. testing showed that the added complexity did not really provide an advantage for the extra horsepower obtained. they also proved to be more maintenance than was necessary.

The R-4360 was installed in the giant B-36 of brief cold war service in the 1950s. I suppose I could use some of the extra energy to run the air conditioner system off of steam , but I could only imagine what a 25 horsepower air conditioner in the passenger compartment would be like…BBBRRRRRR!!!!! The BIGGEST automotive air conditioning system i ever experienced was on our old Buick with those ever wonderful Frigidaire 6 cylinder axial piston compressors that are confirmed to be capapble of absorbing something like 5 horsepower from the engine! Those old AC units would get COLD!!!! and I mean it too!!!! In fact, Having worked with those compressors has compelled me to design my homestead environmental system for heating and cooling and domestic hot water around the old veteran of the automotive AC market! A totally efficient and unified heat pump is what I have in mind for one of those compressors!

I hate to rain upon your parade but one of the reasons that an augmented blowdown turbine is semi practical on an aircraft engine is that the aircraft is typically operated at a relatively high output for much of it’s duty cycle, (read this as damned near full throttle damned near all of the time). Now some try to operate cars and trucks this way but few are successful long term so…

BTW any turbo that tries to extract motative energy from the exhaust stream will typically add to the backpressure of the exhaust and increase the scavenge losses of the engine (remember the tired old phrase “no free lunch”) Another challenge is that piston engines turn in thousands of RPMs and turbines turn in tens of thousands of RPMs and gears used to match the outputs also increase scavenge losses which is exactly why automotive turbines are typically used to pump up the volume of the intake air. Have fun technotwit but also keep in mind that the thermo engineers that preceded you were rather clever too.

urbochargers, blowers and the like increase power by increasing mass per cycle. exhausting to a lower pressure in steam gives you a bigger delta T and so bigger efficiency. I have seen the blowdown turbos on planes, but the speedmatching is a bit of a problem. But the idea of: a) using a steam ejector pump to exhaust to vacuum or b)using a steam cycle to drive auxillaries is pretty novel. the rub that i see with the first one is that water consumption is high. The second one could use water closed loop (with bigger exchangers) and avoid all of the parasitic losses associated with auxillaries.