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Supercritical dieseline as an optimized fuel for low temperature combustion

Transonic demonstrates supercritical gasoline operation under low load, medium load, and high speed low load conditions; simulated vehicle fuel economy of 48.8 mpg

Engineers at Transonic Combustion, a start-up developing a fuel efficient supercritical (SC) fuel injection and combustion system—Transonic Combustion, or TSCi (earlier post)—have demonstrated the TSCi process using gasoline fuel at low load without EGR, medium load with EGR, and high speed low load with EGR. They presented these latest results in a paper at the recent SAE 2012 World Congress, and participated as well in the SAE 2012 High Efficiency IC Engine Symposium immediately preceding the World Congress.

Using engine test data, Transonic simulated vehicle fuel consumption for a vehicle over the NEDC drive cycle using a 4-cylinder, 1.6L engine. Predicted fuel economy for such a vehicle equipped with the TSCi system was 48.8 mpg US (4.8 L/100km): 4.7% better than a diesel engine (46.6 mpg US/5.05 L/100km) and 24.8% better than the vehicle equipped with 2.0L port fuel injected spark ignition engine (39.1 mpg US/6.0 L/100km).

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 NOx 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 NOx (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 NOx 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.

    NOx 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 (COVimep).

  • 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 NOx 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 NOx 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.


  • 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



Their result look very good but they have been around for awhile and it is really a slow process to develop their super-critical injector. I think they first focused on diesel and they now realize that their approach might have bigger benefit on gazoline engine and also might be simpler to implement on gazoline engine as the temperature en pressure to achieve the super-critical state is much lower. anyway will see


This is just electronic inflation madness where they optimise each and every aspect of an engine in labs to try to increase power outputs and more mpg and they didn't improve anything at all since years and years. All these small improvements are obsolete in real world usage where the conditions of lab cannot be reproduced in real life because a normal driver can put more weight in his vehicule like luggages and passengers, hill climbing, cheap gasoline different from region to region, cold, heat, etc.

Engines are studied years after by car manufacturers for each regions and then just adapted or improve a little bit if they discover that some parts are overbuild they they put something cheaper and if some parts are worn too much then they overbuild it a little bit. it take years and years to make small changes.

In a big car compagny just 5% of the engineers take actual decisions put in reality and the 95% left just make reports forgotten the week after and in reality nothing change except tons of useless reports and small amusement experimentations for pr brainwash.

The 100 years old gasoline engines is almost the same as the one of today and the fuel powering it is the same as 100 years ago.



you can't be serious, or how do you explain that a car today pollutes less when it runs than a car parked on a parking lot 30 years ago ?


@ Treehugger , 30 years ago it was the same fuel as today and the same mpg and the same pollution level. The thing is that since that time they do mpg advertisements and emissions publicity and also consumers have to be trained to pay more for 'greener ' conciousness.


I'd like to see a system tailored to handle E85 or M85.  The greater amount of heat recycled to the combustion chamber via the fuel ought to improve efficiency further, and combustion may be cleaner too.


"you can't be serious, or how do you explain that a car today pollutes less when it runs than a car parked on a parking lot 30 years ago ?"

Depends upon what you call "pollution?"
Does your smog tester have the sensor necessary to read a particular "pollutant?"
How about cyanide gas? CN
How about cyanic acid? HCN
All catalytic converters generate CN. But, it must not exist because your smog tester does not have a CN sensor. So, that does not count, right?
That is how modern emissions systems "clean up" the exhaust, they generate CN, which is ignored.
Welcome to the wonderful world of Oz.

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