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Transonic Looking at Final Funding Round This Year, Production Deployment of Supercritical Fuel System in 2014; 50-75% Improvement in Engine Fuel Economy

The use of a supercritical fuel charge enables very fast and complete combustion with minimized thermal losses. The chart shows heat release from a standard diesel combustion cycle (solid black line), overlaid with the heat release from a TSCi cycle (dashed blue line). TSCi combustion minimizes heat loss to the cylinder walls and in the exhaust. Source: Transonic. Click to enlarge.

Transonic Combustion, a start-up developing a supercritical fuel injection system that can improve the fuel economy of internal combustion engines by between 50-75% (earlier post), will raise its final round of funding (D round) this year and is targeting production deployment of its TSCi Fuel Injection System by OEMs by 2014, according to Mike Rocke, Transonic’s Vice President Marketing and Business Development. Transonic was exhibiting at the Department of Energy’s ARPA-E Energy Innovation Summit in Washington earlier this week.

Transonic is currently working with four OEMs on evaluating the system, Rocke said: one each in Asia, North America, Europe and the other in the heavy-duty engine sector.

The TSCi system components. Click to enlarge.

A supercritical fluid is any substance at a temperature and pressure above its thermodynamic critical point. The TSCi Fuel Injection System comprises new fuel injectors that take the fuel charge to a supercritical state just prior to its direct injection at Top Dead Center (TDC) into the cylinder; a next-generation electronic control unit; a high-efficiency fuel pump; and fuel rail (accumulator).

The TSCi injectors can work with a range of fuels (given appropriate software adjustment and control). The company currently is optimizing the system for use in modern high-compression diesel architecture engines, near-term running on gasoline while longer-term utilizing advanced low-carbon fuels.

(Researchers at Syracuse University are working on a method to prepare, inject and combust supercritical diesel fuel. Earlier post.)

Left: TSCi supercritical injection. Right: Liquid Direct Injection. The view is up through the piston bowl. The TSCi charge is nearly invisible to the eye. Click to enlarge.

The TSCi gasoline fuel charge enters the cylinder at around 400 °C—compared to about 100 °C for a conventional liquid direct injection fuel charge—at precisely Top Dead Center (TDC, 0° crank angle). The supercritical charge facilitates short ignition delay and fast combustion, with the energy released focused just on pushing the piston down. The fast combustion minimizes crevice burn and partial combustion near the cylinder walls, and prevents droplet diffusion burn.

The TSCi system supports more efficient engine operation over the full range of conditions—from stoichiometric air-to-fuel ratios at full power to lean 80:1 air-to-fuel ratios at cruise.

The software control is key to facilitating the extremely fast combustion, enabled by advanced microprocessing technology. The TSCi injection system can also be supplemented by advanced thermal management, exhaust gas recovery, electronic valves, and advanced combustion chamber geometries. (The Syracuse team, for example, is using exhaust gas heat to help bring the fuel to SC states.) Through the use of its software, the TSCi system can optimize the use of any combustion chamber geometry or piston bowl shape.

(As an aside to illustrate the flexibility of the TSCi system and the importance of the software control, Rocke said that during one of their demonstrations to an OEM, the Transonic engineers switched from diesel to gasoline in the fuel supply. Although the switch required some rapid on-the-fly computer keyboard work by an engineer, the engine didn’t miss a beat, according to Rocke.)

Transonic’s testing on a mid-size vehicle on a chassis dynamometer resulted in EPA highway fuel economy of 64 mpg (3.7 L/100km), with city testing being finalized this year, estimated at 47 mpg (5.0 L/100km). As a comparison, the 2010 Prius delivers 51 mpg US (4.6 L/100km) EPA city and 48 mpg (4.9 L/100km) highway. The TSCi system would add about $1,500 to the cost of the vehicle, Rocke said, compared to about a $4,000 delta for a current full hybrid solution.

The Transonic OEM system can be deployed on existing engine architectures with minimal OEM re-configuration, a “relative drop-in” solution.

Although its early work showed significant reductions in engine-out emissions, Transonic will begin more thorough emissions testing on its system this year, according to Rocke.



For anyone from Transonic, please answer the following questions:
-Are your measured fuel economy gains from a vehicle that demonstrated emissions compliance? if so are they at or below SULEV?
-Is the test vehicle used for testing a production vehicle? What is its mass, Cd, and frontal area? How quickly can it accelerate to 60 mph?
-How do you meet or plan on addressing NOx emissions requirements when running lean? Do you use a LNT, or SCR system?
-What is the highest peak cylinder pressure you see?
-What is the cylinder pressure rise rate in Bar/degree crank angle?
-Are you running pump or laboratory spec gasoline? How do you results change with blended fuels (E10-E85)

Thanks, Best Regards, and Good Luck


Looks like a system that would need exhaust heat to bring it up to operating temperature, might be difficult to get it working over different conditions when its working as a conventional ICE, but if it was running as a range extender, fixed speed under a fixed load it would be easier to control.

Account Deleted

It's a brilliant concept. I did something similar years ago and the findings can be found in SAE 2009-01-2808. As liquid is compressed, both specific heat capacity and boiling point increases. When this happens, the liquid that is about to be injected can absorb a lot of heat without boiling. By injecting it after it absorbs the exhaust heat, we can actually transport the thermal energy back into the engine.


I understand the benefits of exhaust heat recuperation, but my estimate is that this is a relatively minor change in enthalpy relatively to the energy released during the combustion process. Can you comment as to what you found? Also, how significant is this aspect relative to the benefit of better combustion phasing or lean operation? This is where there is low hanging fruit in an S.I. engine., but as I eluded to above, unless the emissions and NVH challenges are addressed, I question its large scale commercial viability.

Roger Pham

The 64 mpg value is probably for a Tdi diesel vehicle, which probably is making already above 55 mpg hwy rating before the TSCi modification.

It would be unlikely that TSCi can improve on the efficiency of an HEV engine on Atkinson cycle running at 1600-2000 rpm, which is the typical engine speed for hwy cruise at 60-70 mph. At that speeds, there is plenty of time for combustion to occur, and at the higher manifold pressures typical of the HEV engines at cruise (higher charge density), spark ignition on homogenous charge creates rapid flame propagation and rapid enough combustion that would be difficult to improve upon.

TSCi does not have the charge-cooling effect of GDI, which is magnified by alcohol inject at high boost.

The main advantage of TSCi is to permit diesel engine to operate at higher rpm's, hence higher power output per displacement unit which will help truckers a lot, also some moderate improvement in efficiency at cruise, but probably not a whole lot, since large-bore diesel engines already running quite slowly (1,300-1,500 rpms) at cruise, giving enough time for combustion via conventional diesel injection. The use of much higher injection pressures in diesel engines nowaday gives much finer particles that should negate much of the advantage of the TSCi concept.

Account Deleted


You're right, considering that fuel is at small quantity, there is only small amount of energy that we can bring into the system actually. That's why I decided to do the supercritical thing to water instead of fuel. By injecting around 9.5 times of water at 320C relative to 1 mass of fuel, we can actually supply thermal energy equivalent to the chemical energy of 1 mass of fuel.

Here's my take on the supercritical fuel, when diesel fuel is injected, it will first have to 1) atomize, 2) vaporize 3) get itself close to oxygen molecule 4) auto ignite.

By going into supercritical, it stays in liquid state because the injection pressure is high. However, as it is injected, the pressure in the combustion chamber is lower thus it boils immediately. This considerably shorten the ignition delay.

If you have a chance, read my work in the SAE paper, it's a long one though


Can you precise, Azmi, what kind of molecules you will obtain at the exhaust line output, if you use an internal combustion of H2O (in SC State) in combination with the air. The classic air contains 19% of Oxygen but also 80% of N2. Don’t we risk seriously woldwide expanded pollutions due to massive SC chemical reactions results in theirs forms like HNO3, NOx, O3,etc...?

Account Deleted

if you read my paper, I purposely eliminate the emission formations by replacing air with oxygen. With no nitrogen during combustion, we basically get very high purity CO2 and some water. Not only that, I stratify the oxygen and fuel inside the piston bowl to ensure very short ignition delay, low soot and HC formations.

By now, you must be wondering what is the purpose of supercritical water, well with the oxygen combustion, water cooling is a must. To speed up water vaporization, I introduce water in at 320C where the relatively lower combustion chamber pressure turns it almost immediately to water vapor. Water's high specific heat capacity in both gaseous and liquid forms ensures excellent cooling inside the combustion chamber.

Once in water vapor, it expands few times more than nitrogen to push the piston down (in piston engine) and turbine blades (gas turbine).

I basically revive decades old research by the US Army tank command;
by improving almost everything from the earlier research. Considering that most of the fossil fuels are consumed by either piston or gas turbine, i think it's time for us to replace the current heat engines with new ones.


I looks like a pretty good concept. I am wondering about NOx emissions too, are they expected to be lowered too that way ? As there is no premixed phase anymore...

Azmi : I would be interested to read your paper. Using Oxygen instead of air would lead to a very high combustion temperaure, so I think that s why you use water cooling. But I would you feed the engine with pure oxygen for a non steady application ? With high pressure bottles of O2 ? In that case, what would be the expected range of such a car ? For instance, a truck engine of 150 kW can use around 1200 m3/h of air, so around 240 m3/h of 02...

May be the answer is in the paper.

Account Deleted

assuming that we get the oxygen from PSA, the impurities are mostly water and argon, thus there wont be much nitrogen to form NOx. With less NOx to worry about, I can push the combustion temperature above 2000K for better combustion efficiency.

For typical passenger car, yes we need a composite tank that is normally used for CNG. Oxygen has about twice the density of methane thus we can pack a lot of oxygen in the same tank.

My target range of travel for every combined filling of oxygen, water and fuel will be 400 km. I made some calculation but it is not achievable with diesel. A better choice will be methanol as it is an oxygenated fuel. I reckon that at high combustion temperature, the oxygen portion from the methanol fuel can be fully liberated to take part in the combustion. Depending on the oxygen purity use, using methanol, the air fuel ratio is close to 1 to 1 ratio, so we dont really need to carry lots of oxygen in the composite tank.

I basically rely a lot on the high efficiency of the engine to ensure that both oxygen and fuel are minimally used to travel the same distance. With the engine requiring no radiator and with the exhaust temperature lower than 50 C, I can bet my money that the thermal efficiency is high enough for the car to travel far with less fuel and oxygen.

For main battle tank, big truck, ship and electric power plant, you can use onboard oxygen generator and there is no need for compressed oxygen anymore.


Sorry when I said : "I am wondering about NOx emissions too" I was talking about the Tsci concept, not your project. I know that with no N2, there is no way to create NOx !

About yours now : have you run an adiabatic flame temperature calculation (chimical thermodynamic), I am pretty sure that with pure oxygen and poor inert gas, it could go goes pretty high, more than 2500 K, and then cooling of engines parts will be a big issue. That's also one reason why you'll still need a radiator to take care of your heat rejection through combustion process (even with cooled exhaust gases).

Also, methanol stochiometric combustion is 6.5:1 I think, why are talking about 1:1? Do you mean, equivalent ratio of 1, as in most gasoline engine ?


Ok sorry, I guess you meant oxigen to fuel ratio, not air to fuel.

Account Deleted

with no cooling agent, the combustion temperature went up to 2800K. It would melt the piston even if I raise the silicone content of the alloy

As for the radiator, yes I do need the coolant to cool off the top part of the cylinder bore and some part of the combustion chamber flame face. However, instead of using the outside air to cool off the coolant, i use heat exchangers to heat up water prior to the injection into the engine.

if you read the paper from the US army, they didnt use any radiator at all.

Account Deleted

yup, it's supposed to be oxygen to fuel ratio and it's not really 1:1 it's more like low 1.2-1.5. The stratification of oxygen and fuel in the piston bowl avoids the need of running it lean.


Well it looks like Transonic won't be making a that case let me answer my own questions...the car they are using is a kit car that seats two people with unrealistic CD and frontal area. The engine is a small european TDI engine that doesn't provide vehicle performance that most consumers expect and demand. The engine has not demonstrated emissions compliance and with lean operation, it will not meet the NOx limits in most developed nations without expensive and bulky NOx aftertreatment that consumes a Urea water mixture that needs filling every few gasoline tanks. Because they are running compression ignition on gasoline, the pressure rise rates far exceed what OE's and their customers tolerate from an NVH perspective. All of these issues can be resolved but not without compromising on efficiency, which is more or less true for conventional engines. I think the concept has merit, but the benefits are much smaller than claimed and it isn't clear that they will justify the cost and complexity.

Sorry Khosla...try again.

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