Lund Team Shows 57% Thermodynamic Efficiency in a Gasoline-Fueled Heavy-Duty Diesel Engine Using PPC
|Gross indicated efficiency (%) of Scania heavy-duty diesel running on gasoline using PPC. source: Bengt Johansson. Click to enlarge.|
Researchers at Lund University in Sweden have shown a thermodynamic efficiency of 56% in a gasoline-fueled single-cylinder light-duty engine and 57% in a gasoline-fueled single-cylinder heavy-duty engine using the Partially Premixed Combustion (PPC) concept. Under higher load, they achieved 52-55% thermodynamic efficiency with 99.8% combustion efficiency and engine-out NOx below US10/Euro6 levels in the heavy-duty engine.
Prof. Bengt Johansson presented the results at the US Department of Energy’s 16th Directions in Engine-Efficiency and Emissions Research (DEER) Conference in Detroit.
Although HCCI (Homogeneous Charge Compression Ignition) was developed as a means to achieve ultra-low NOx and soot simultaneously with higher efficiencies, the viability of HCCI applications appears limited to lower load operations. Among the challenges facing HCCI combustion are acoustic noise, lack of direct control of the combustion and too diluted air-fuel mixture requirement.
Partially Premixed Combustion (PPC) was developed as a means to increase efficiency in the order of 50% or above, achieve low NOx and soot, and run the whole load range. PPC is a mix of the classic diesel combustion process and HCCI using a fully pre-mixed charge. If you consider standard diesel combustion as “black” and HCCI as “white”, Johansson said, PPC is some varying shade of gray.
PPC uses a rather early injection to create a premixed fraction of the fuel mixture, and a late injection to obtain stratification. By changing the ratio of these two injections, it is possible to tune the burn rate.
More fuel in the first injection means a faster, more HCCI-like combustion; more fuel in the second injection results in combustion more like regular diesel diffusion controlled combustion.
The Lund team worked with a Saab variable compression ratio engine, a GM L850 world engine, and the Scania heavy-duty diesel engine. As a baseline, they found that running the GM engine in spark ignition (SI) mode at a compression ratio of 9.5:1 (standard) at low load resulted in a thermodynamic efficiency of about 30%. Boosting the compression ratio to 18:1 resulted in thermodynamic efficiencies in the range of 30-40%. Switching combustion mode from normal flame propagation to HCCI resulted in roughly 50% thermodynamic efficiency across the three engine platforms, Johansson said.
We can also see all three engines would have about the same thermodynamic efficiency. There is no major difference...We can also see that HCCI kind of stops at about 6 bar BMEP. We did 20 bar HCCI, but I would not recommend it...There is actually a [combustion efficiency] penalty going from SI to HCCI. People saying that HCCI is an efficient combustion process, I wouldn’t really say so because you lose 10% of the fuel.—Bengt Johansson
When using a diesel (compression ignition) engine running with diesel fuel, the load region in which the engine can run in PPC mode is limited to 5-6 bar gross IMEP, noted Vittorio Manente from Lund in his PhD thesis on the subject. Increasing the upper load PPC boundary in a diesel engine could thus either require a piston with much lower compression ratio has in conjunction with “an intolerable amount of EGR” to keep the start of combustion and the end of injection separated; or a fuel that is more resistant to auto ignition needs to be used: e.g., low-cetane diesel or gasoline.
The Lund team used regular US gasoline in their engines.
Bengt Johansson et al., Path to High-Efficiency Gasoline Engine (DEER 2010)
Bengt Johansson (2009) Partially Premixed Combustion, PPC, for high fuel efficiency engine operation (SAE 2009 Powertrains, Fuels and Lubricants Meeting)
Vittorio Manente (2010) Gasoline Partially Premixed Combustion: An Advanced Internal Combustion Engine Concept Aimed to High Efficiency, Low Emissions and Low Acoustic Noise in the Whole Load Range (PhD Thesis)
Bengt Johansson (2005) High-Load Partially Premixed Combustion in a Heavy-Duty Diesel Engine (DEER 2005)