Transonic Combustion injection system shows 5-21% fuel consumption reduction compared to PFI engine; simultaneous reduction in NOx and PM at high EGR rates
|Normalized fuel consumption reduction from TSCi over the mini-map operating points compared to production PFI SI engine. Click to enlarge.|
Engine testing of the Transonic Combustion injection-ignition system (TSCi, earlier post) applied to a gasoline-fueled production passenger car engine (with a diesel engine architecture) has shown fuel consumption reduction in the range of 5% to 21% across a number of speed and load conditions compared to a production port fuel injection (PFI) spark-ignition (SI) engine.
Modeling results using 1-D gas dynamics at 1,500 rpm and a load of 3 bar IMEP showed thermal efficiency improvements in the range of 21.6% and ISFC reduction of 16% in comparison with an SI gasoline engine part load. Chris de Boer, Transonic’s vice president of research and development, presented the results in a paper at this week’s SAE 2010 Powertrains Fuels & Lubricants meeting in San Diego. (This marked the start-up’s first public technical presentation, he noted.)
|TSCi prototype injector. Click to enlarge.|
The TSCi system features direct injection of fuel as a supercritical fluid, which is injected later in the compression stroke using a proprietary injector. The basis of the combustion process is the delay of the injection of the fuel to the extent that the heat release predominantly takes place after TDC of the engine power stroke. To be able to achieve this, the combustion process must have a short delay period, followed by rapid air-fuel mixing and combustion—characteristics that Transonic says are achieved by injecting the fuel as a supercritical fluid.
In his presentation, de Boer noted that while gasoline SI efficiency is constrained by thermal efficiency, throttling and compression ratio, the Transonic system has low heat losses, unthrottled operation, and a high compression ratio.
Multiple ignition sites and rapid combustion with TSCi combine to result in high rates of heat release and high cycle efficiency. The injection-ignition process is independent from the overall air/fuel ratio contained in the cylinder, allowing the engine to operate un-throttled. Additionally, the stratified nature of the charge under part load conditions reduces heat loss to the surrounding surfaces, resulting in further efficiency improvements, Transonic says.
The short combustion delay angles allow for the injection timing to be such that the ignition and combustion events take place after TDC. The advantage of the late injection timing is that all work resulting from heat release produces positive work on the piston. Other advantages are the elimination of droplet burning and increased combustion stability that results from multiple ignition sources.
In the modeled results that showed a 21.6% improvement in thermal efficiency, 12.5% was due to the reduction of pumping losses, 5% from the more effective heat release diagram, and 4.1% from the change in compression ratio, de Boer said.
The characteristics of TSCi address all of the issues identified above as limiting the efficiency of the gasoline engine; it is capable of operating over a wide range of air/fuel ratios and so does not require a throttle for load control. TSCi has inherently short combustion delay and fast combustion that combine in heat release phasing for optimal efficiency. TSCi can be operated at an optimal compression ratio since it is not dependent on high octane gasoline. The ignition mechanism, discussed in more detail below renders the combustion system fuel neutral in the sense it is not reliant on either Octane or Cetane values.—de Boer et al.
|A supercritical fluid is any substance at a temperature and pressure above its critical point. Supercritical fluids generally have properties between those of a gas and a liquid, as well as other properties such as having no surface tension, the ability to solvate other liquids and solids, and the formation of small particles with a narrow size distribution during a phase change to liquid.|
|Supercritical fluids possess rapid mass transfer properties with diffusion coefficients more than ten times that of a liquid near the critical point, de Boer et al. note. The density ranges between one third and two thirds of that of the corresponding fluid and varies significantly with temperature and pressure.|
|“Gasoline is a blend of C4 to C12 hydrocarbons, contains hundreds of different molecules, and varies in composition. The critical point for a mixture of compounds is difficult to predict exactly, however, indications are that the Tc and Pc for a mixture will be lower than that of its constituents.|
|“...The ignition characteristics of supercritical gasoline are not well documented and are the subject of current detailed experimental and modeling activities. Results from this work will be the subject of future publications.”|
|—de Boer et al.|
The experimental engine was a 4-cylinder, 1.56 liter unit with a compression ratio of 17.4. Baseline engine data was established from two US 4-cylinder SI gasoline engines in the two liter class and featuring VVT.
Adapting the experimental engine for TSCi required only a minor modification to the centrally located injector location to adapt it to the design of the TSCi injector geometry. No modifications were made to the combustion chamber, air motion level or compression ratio of the base diesel engine.
A TSCi injector was installed in each cylinder without change to the clearance volume. The TSCi injector has a multi-hole nozzle with an included injection angle of 120°. Orifice configuration and geometry were fixed for these experiments; no optimization was carried out. A fuel pump having a pressure delivery range of 200 to 300 bar was used to pressurize the fuel—an 87 octane gasoline—in a common rail.
The pressurized fuel is heated using a combination of exhaust heat recovery and electrical heating. The design of the injector is such as to facilitate heating of the fuel at the point of injection. An injector down-tube temperature of 370°C and fuel pressure of 20.5 MPa was used for all test conditions reported in this paper. At this fuel pressure and injector temperature the conditions are such to ensure that the fuel components that make up gasoline are in a supercritical state, this has been confirmed by thermal bench testing. At the current stage of design and development of the TSC injection system there is a detailed understanding of the energy required to heat the fuel and the interaction with overall fuel efficiency. The system design is continually improving and the impact of the fuel heating requirements is already neutral to the overall efficiency of the engine.—de Boer et al.
Transonic varied EGR from 0% to 48% in its engine testing. The TSCi combustion system exhibits high EGR tolerance considering the light load operation, they found. The tolerance level allows the fuel consumption to be reduced by 4% with up to 40% EGR.
On the emissions side, the testing showed a gradual reducing HC trend with increasing EGR rate. However, CO emissions show a gradual increase with increasing EGR rates, suggesting partial combustion. Transonic calculated a combustion efficiency of above 98% throughout the EGR range based on the HC and CO levels.
NOx emission decreases as adiabatic flame temperature decreases as a result of the increased mixture dilution with EGR. At high EGR rates, a very low level of NOx emission down to 0.2 g/kWh is achieved. This emissions level is similar to that measured with HCCI combustion systems. Smoke is seen to simultaneously reduce with NOx. Considering the flame equivalence ratio and flame temperature relationship with NOx and Soot formation (Φ-T map), the reduced local flame temperature effectively avoids both soot and NOx formation. With increasing EGR rates injection timing was advanced in order to preserve optimum combustion phasing and efficiency.—de Boer et al.
In the question and answer session following the presentation, de Boer noted that while that not all aspects of the fundamentals of the new combustion system are fully understood at this point, the initial application of the TSCi combustion system has provided a foundation for ongoing R&D for further optimization of the technology.
Chris De Boer, Junseok Chang and Shreeram Shetty (2010) Transonic Combustion - A Novel Injection-Ignition System for Improved Gasoline Engine Efficiency (SAE 2010-01-2110)