A series of hybrid Q&A bullets to shine a bit more light on BMW's Turbosteamer and Scuderi's Split-Cycle engine.
Motor Trend, May 2006
Motor Trend Magazine's June 2006 Technologue column introduces two pretty neat alternative hybrid schemes. Inquiring minds are bound to have questions about how each works, so here's a series of Q&A bullets to shine a bit more light on BMW's Turbosteamer and Scuderi's Split-Cycle engine.
BMW Turbosteamer
Why are there two temperature circuits?
It takes two circuits to squeeze all the heat out of the exhaust and to drop its temperature by over 1000 degrees. The circuits are separated, and run fluids with different boiling points.
What are the liquids used to generate the steam?
The low-temp circuit uses ethanol, which boils at around 160 degrees, and sheds its heat using a coolant-to-air radiator. The high-temperature circuit uses a water-based fluid similar to engine coolant, and it sheds its heat to the low-temperature circuit's ethanol fluid so as not to waste it.
What temperatures and pressures are we talking about here?
The high-temp circuit runs at up to 1000 degrees, the low-temp side peaks at around 300 degrees. Steam pressure runs between 150 and 600 psi.
How does the catalytic converter stay lit with all the heat being extracted?
The superheater that powers the high-temp circuit is located aft of the pre-catalyst and before the main unit, which still gets enough heat to clean up the emission that escape the pre-cats.
Where does the low-temp circuit get its heat?
Ethanol that is already steaming after condensing the high-temp circuit is then superheated in an exchanger just ahead of the muffler. This lower-temperature exchanger also pre-heats the coolant in the high-temperature circuit. Together they remove enough heat so that you can hold your hand under the exhaust pipe without getting burned.
As exhaust-system temperatures vary with operating conditions, how is the steam circuit output matched to engine demand?
BMW has not yet disclosed the control strategy, but we understand it works something like a variable-output air-conditioning compressor, with a swash-plate or other means of varying the compressor's stroke, and hence its output.
When can we see it, and on what cars?
BMW doesn't expect the Turbosteamer to hit production for about a decade, but at that point it could be applied to all but the smallest of its products, where packaging concerns could present a problem.
How much will it cost?
That's anybody's guess at this point, and too tough to call.
Scuderi Split-Cycle Hybrid Engine
First a recap of the basics: The engine employs pistons and connecting rods turning a crankshaft, just like 99.9 percent of the engines on the road, but here half of them breathe in and compress the air, while the other pistons combust and exhaust the mixture. The basic engine's overall efficiency is estimated at 43 percent (up from about 33 percent for ordinary engines), with a compressed-air hyrbrid system boosting that to over 50 percent.
Why split the cycles?
The primary benefit is that now the cylinder designs can be optimized for the two fairly different tasks of compressing and combusting.
How do the compression and combustion cylinders differ?
By altering the bore and stroke slightly, it's possible to have very different compression and expansion ratios--one of the holy grails of engine performance and efficiency. The compression cylinders can have very high compression, because there is never any fuel in them to detonate. A larger bore also permits a natural supercharging effect of compressing a larger volume of air than the combustion chamber displaces. (Air pressures of up to 725 psi have been suggested.) A longer stroke on the combustion cylinder extracts more power from the charge. In this way, the Split-Cycle concept mimics the Miller-cycle.
How does the air get from one cylinder to the other?
There's a short passageway connecting the cylinders. A conventional cam-actuated valve admits air to the combustion chamber, but a pressure-activated disk-type valve (picture a heavy-duty version of the reed-valves in your Briggs & Stratton) admits air into the passageway from the compression chamber and prevents it from leaking back into the intake cylinder.
When and where is the fuel injected?
Fuel is injected directly into the cylinder just as the valve opens. The highly-compressed air whooshes in so quickly and atomizes the injected fuel so thoroughly that the spark is fired to ignite it 11 to 15 degrees after top-dead-center, even when running lean mixtures for increased efficiency.
How on earth can the flame burn completely with such late ignition?
Conventional wisdom holds that, particularly at higher engine speeds, the spark must be advanced to light the mixture well in advance of top-dead-center in order to have enough time to fully burn the mixture before the bottom of the piston's stroke. Scuderi's modeling suggests that the high air pressures and turbulence serve to atomize the fuel so thoroughly that the mixture burns in about half the time it takes in a conventional engine.
What about emissions?
Lighting the mixture while the piston is still compressing it creates extreme heat and pressures that cause the formation of NOx, which ordinary engines combat by recirculating exhaust gasses and using a three-way catalyst. By lighting the mixture when the piston is already moving down, the flame front actually cools as it propagates, reducing the production of NOx in the cylinder by a claimed 80%. It's not low enough to meet current standards, but it requires less aftertreatment and no exhaust-gas recirculation is needed.
What about friction?
Another benefit of splitting the cycles is that each side of the engine only loads the crank heavily in one direction (up for the compression side, and down for the power side). This means that the cylinders can be offset from the crank so that power is being transferred to the crank (or from the crank to the compression piston) at the two points when the connecting rod is vertical. There is little or no side-load on the piston at these points, which typically occur at the very top and bottom of the stroke in engines that center their cylinders over the crank, and no work happens at top and bottom dead center.
How does splitting the cycle and offsetting the cylinders affect engine balance?
The forces at work in the Split-Cycle engine are slightly different than those of a typical engine of similar cylinder count, but the fact that most of the cylinders are moving in the same direction most of the time actually simplifies the task of balancing somewhat. Conventional crankshaft counterweights and balance shafts are expected to manage the balancing act.
How does the hybridization work?
Simple: Take your foot off the gas and the fuel is shut off. The compression pistons continue to pressurize air, but valves admit the air to a storage tank before the valve opens to the power cylinders. Get back on the gas, and an electronically-controlled pressure-activated disc valve on the inlet side of the compressor cylinder opens so there is no load on that piston. Air is supplied to the power cylinders from the storage tank, increasing thermal efficiency to 70 percent while the compression pistons are offline. There is also talk of using the stored air to operate pneumatic valvetrain with infinitely variable timing and lift, but that's a phase-II or later innovation.
So where do the savings come from?
Computer modeling conducted by the Southwest Research Institute suggest that, relative to a conventional engine (of 33.2% thermal efficiency), the Split-Cycle gains 5 percentage points from the faster burn rate, 2 percentage points from using ceramic linings to retain heat in the power cylinders, 1 percentage point from running lean without needing a three-way catalyst, and another 1.4 percentage points due to the unique piston motions. That brings the un-hybridized engine to 42.6 % thermal efficiency. The hybrid hasn't been thoroughly modeled yet, but another 10 points or so are expected.
Isn't this just another crackpot odd-ball engine with no real future?
The Department of Defense doesn't seem to think so. Maybe it was simple pork-barrel politics as usual, but Scuderi has just been awarded $1.2 million to further the development of its engine based largely on the Southwest Research Institute's findings. The fact that the engine can be produced on conventional tooling and mated to normal transmissions also bodes well for its chances of eventual production.
What about cost?
It's way too early to tell, but savings from less expensive catalysts might offset the cost of ceramic cylinder linings, and it would seem likely that the disc valves could be produced for the same or less money than another camshaft or two and a series of poppet valves. But we'll obviously have to keep a close eye on this aspect of the project. Stay tuned...
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