Scuderi working on turbocharged variant of split-cycle engine; smaller, more powerful, and more fuel efficient
|Cartoon of the proposed turbocharged split-cycle engine. Note the reduction in size of the intake/compression cylinder. Click to enlarge.|
The Scuderi Group announced at the SAE 2011 World Congress in Detroit that boosting one of its air-hybrid split-cycle engines (earlier post) with a turbocharger to 3.2 bar decreases the BSFC (brake specific fuel consumption) up to 14% compared to the air-hybrid engine, while resulting in a simultaneous increase in the engine’s power BMEP (brake mean effective pressure) by 140%. At the same time, the engine can be reduced in size by roughly 29%, according to recent modeling results.
The basic Scuderi engine divides the four strokes of a combustion cycle among two paired cylinders—the left cylinder functions as an air compressor, handling intake and compression, while the right cylinder handles combustion and exhaust. Key to Scuderi’s split-cycle design is that it fires after top dead center. An air-hybrid configuration of the engine adds a compressed air storage tank. (Earlier post.)
A naturally aspirated Scuderi Air-Hybrid configuration consumes 30-36% less fuel under similar drive conditions, according to preliminary results from simulations released earlier this year. (Earlier post.)
|Rendering of the turbocharged split-cycle engine. Click to enlarge.|
Consistent with conventional four-stroke engine designs, the combustion cycle of the Scuderi Engine has two high-pressure strokes—compression and power, and two low-pressure strokes—intake and exhaust. The power stroke is positive work, or the energy that is produced by the expanding gases to create mechanical work. The intake, compression and exhaust strokes are all negative work, or the energy that the engine consumes to create mechanical work.
With the compression cylinder separated from the power cylinder, the use of a standard turbocharger to convert recovered exhaust-gas energy into compressed air energy supports the downsizing of the compression cylinder to achieve substantial reductions in negative compression work.
The amount that ends up on the crankshaft is the difference between the negative work and the positive work. And when you split the cycles like this you can now try to figure out ways to reduce that negative side without impacting the power side.
We determined that if you put a simple standard turbocharger onto the engine, and you feed more air into the compression side—now it’s coming in at higher mass flow and high pressure—when you do that on a normal engine, what happens is that the pressures in that compression stroke goes up. Your power goes up, but you’ve pushed more mass into the cylinder and when you squeeze it you’re getting a higher pressure. Now, when you fire, you’ve got more air at a higher pressure, you’ll get more power out, but the efficiency doesn’t go up. In other words, you’ve got more power, but you’ve also have more negative work going in.
If we control the pressures of the engine internally so that you are not causing more pressure to occur, what happens is you cause less work of compression to come off the crank. If I boost it to say 2 bar, I have twice the mass of air. Now when I compress to get to my naturally aspirated compression levels, I only have to boost halfway, because we normalize. We don’s let the pressures go up to where they would normally be. So you literally do less compression work for the crank. In other words, the energy off the exhaust is actually doing some of the compression work for us.
By doing that, the volumetric efficiency of the compression side goes way up. When you turboboost to 2 bar and you only compress to a naturally aspirated level, the flow of air is twice what you need. You can either feed [two expansion cylinders] or the logical, more simple approach is that you downsize the compression cylinder. Even though it is downsized, it’s still feeding the amount of air you need. The net effect is the compression work goes down, the engine gets smaller and the efficiency gets better.—Sal Scuderi, President of Scuderi Group
|Simulated power and fuel consumption at 1400 and 4000 rpm. 40% turbo efficiency. Click to enlarge.|
Scuderi said the that company is just beginning to explore the potential of the turbo design. The Group is in discussions with 15-16 OEMs so far, Scuderi said, and is under non-disclosure with 11. Scuderi is performing simulations in vehicle for three OEMs. Scuderi expects its first license by the end of the year.