The TSCi combustion process has similarities with gasoline partially premixed combustion (PPC), homogeneous charge compression ignition (HCCI); reactivity controlled compression ignition (RCCI) and low temperature combustion (LTC) with high indicated thermal efficiencies of greater than 45% and simultaneous reduction of NO_x and smoke at high EGR levels.

To reach SC state, gasoline is heated above 280 °C by the TSCi injector and pressured to greater than 42 bar; this decreases the density of the fuel by up to 50%, and improves the diffusivity of the fuel by an order of magnitude, according to Transonic. The Transonic injector has larger nozzle hole diameters and internal volumes than conventional diesel injectors, enabling sufficient flow of lower density heated fuel.

We inject gasoline under supercritical conditions to provide potential for improved efficiency. The spark ignited gasoline engine is constrained by compression ratio, fundamentally because of spark-knock limitations. The spark ignition flamefront also results in slow burn, and that means we have high heat losses and inefficient heat release profile. And at light load, we have pumping losses due to throttling.

We operate at high compression ratio, we have a compression-ignition combustion system which results in fast burn. This enables us to idealize the heat release profile, and we have very good lean limits so we can run essentially unthrottled. The Transonic combustion systems addresses the problems that are limiting gasoline efficiency.

—Chris de Boer, VP Research & Development, Transonic Combustion at SAE High Efficiency IC Engines Symposium

Prior work at Transonic had shown high indicated thermal efficiency (up to 42%) with the use of EGR, and SC fuel temperature at 2000 rpm and 2 bar BMEP on a 1.6L 4-cylinder CI engine. When EGR was swept from 0 to 43% with fuel temperature at 280 °C, the result was low brake specific NO_x (0.5g/kWh), negligible smoke emissions, and low HC and CO emissions. Increasing the fuel temperature resulted in an improvement of premixing, which improved combustion efficiency from 95% to 99%.

The benefits of TSCi combustion over Gasoline PPC for improving premix are apparent. However, further work is needed to characterize the effects of TSCi combustion process at low speed and low load and high speed low load where combustion stability and fuel economy pose limitations for Gasoline PPC. Additionally, extension of TSCi combustion process to medium and high operating ranges is needed, where pressure rise rates and smoke emissions become the dominant factors.

—Zoldak et al.

For the work described in the paper, Transonic used a single cylinder test engine based on an existing light-duty diesel powertrain from Mahle Powertrain. The test setup used the cylinder head, camshafts, piston, connecting rod and liner form the production engine, and include a TSCi fuel system in place of the common rail.

TSCi injector version 2.0 featured in the study. The fuel was pressurized by a Transonic positive displacement pump capable of variable pressure control and preheated to 250 °C prior to reaching the injector. In a multi-cylinder application, the fuel preheating will be accomplished using waste exhaust heat. The fuel system can handle low lubricity fuels such as gasoline with up to 10% ethanol.

A high pressure (HP) loop EGR system was used with an EGR cooler and a hot-side EGR valve to control EGR mass flowrate; the system was capable of handling up to 60% EGR. All tests used commercially available California regular gasoline with an 87 pump octane number.

Among the overall results and findings from the study were:

• Low load operation was shown at 1000 rpm 1.6 bar IMPE with fuel temperatures in the range of 200 to 300 °C, a boost level of 106 kPa absolute and an intake manifold temperature of 41 °C.

  The fuel temperature was effective in controlling NO_x emissions without the need for EGR. Combustion stability and fuel economy were optimized with fuel temperature of 250 °C and SOI timing of -18 °ATDC, controlling the level of premix via stratification.

  NO_x levels were 2.81 g/kWh (77ppm), HC and CO were 16.9 g/kWh and 61.4 g/kWh, fuel consumption was 248.1 g/kWh and combustion stability was 3.1% coefficient of variation of IMEP (COV_imep).

• Medium load was demonstrated at 2000 rpm 7.6 bar IMPE with optimization of several parameters, including SOI, EGR, boost, fuel pressure at rail and fuel temperature setpoint.

  The optimum point for this load condition showed NO_x level of 0.73 g/kWh, smoke of 0.53FSN, HC at 4.43 g/kWh and CO at 10.32 g/kWh. This resulted in a rate of pressure rise of 12 bar/deg and 45% indicated thermal efficiency, enabling a fuel consumption of 185 g/kWh.

• High speed low load operation was shown using SC fuel temperature, hot intake manifold temperature and EGR. The optimum point for 3000 rpm 3.4 bar IMEP showed NO_x level of 1.02 g/kWh with smoke of 0.17FSN, HC of 5.4 g/kWh and CO of 18.1 g/kWh. Indicated thermal efficiency at this point was 43.5% with fuel consumption at 191 g/kWh.

Resources

• Philip Zoldak, Chris de Boer and Shreeram Shetty (2012) Transonic Combustion - Supercritical Gasoline Combustion Operating Range Extension for Low Emissions and High Thermal Efficiency (SAE 2012-01-0702) doi: 10.4271/2012-01-0702