Petro-diesel combo engine shows promise

Scientists from the University of Wisconsin have come up with a rival for Daimler's DiesOtto concept, combining petrol and diesel combustion in the one engine. Benz, meanwhile, has gone notably quiet on DiesOtto.

Burning petrol and diesel fuels in a single engine is a tricky business. Beyond the essential deployment of an internal combustion cycle to convert the potential energy contained in fossil fuels first to heat and then to kinetic energy, there's little in common between the way diesel and petrol engines work. Different fuel chemistries make for different combustion characteristics, delivering different benefits and shortcomings.

Many of these make the two types of combustion complementary. The oiler delivers higher thermal efficiency. That is, it loses less of the fuel it consumes in heat transfer (that's the heat you feel from under the bonnet when an engine's been running) and rear-end emissions, turning a higher percentage of the energy it produces into movement. It therefore uses less fuel, but what does come out in exhaust is considerably dirtier.

Petrol, being thinner and more volatile, burns faster than diesoline. More of it is consumed on the bang-stroke of the combustion cycle, so making the petrol engine intrinsically cleaner at the exhaust end.

Engineers have long noted the considerable benefits to be gained in an engine capable of using both fuels in a way that capitalises on the benefits of each. Mercedes-Benz's people have been working on the idea for some time. The resulting 'DiesOtto' debuted in 2007's F 700 research vehicle. That odd name hails from a mix of 'diesel' with 'Otto' -- the former derived from Rudolf Diesel, inventor of the eponymous engine and the latter coming from Nicolaus Otto, inventor of the 'Otto cycle' four-stroke petrol engine.

Now, a mechanical engineering research team from the University of Wisconsin has come up with a new take on the idea. This one uses a fuel injection system with the reflexes to optimise the diesel-petrol blend in real-time, in the combustion chamber.

The system borrows from Rudolf Diesel's idea of dispensing with external spark ignition and using compression to trigger combustion instead. That's where the diesel fuel comes in. "You can think of the diesel spray as a collection of liquid spark plugs, essentially, that ignite the gasoline," says research team leader Rolf Reitz.

From there, it varies the fuel mix according to criteria like driving conditions, driver behaviour and load. In a truck under heavy load, the engine might skew the fuel mix to 85/15 petrol-to-diesel. Lighten the load and the proportion of diesel would rise to around 50/50. Under normal conditions, petrol's higher flash point would mean such a blend would fail to ignite.

But the right proportion of diesel fuel would give it the kick it needs.

The results look promising, starting with a boost in fuel efficiency averaging 20 per cent. Combustion temperatures up to 40 per cent lower mean dramatic reductions in energy loss through heat transfer. And because it's constantly shifting the fuel mix to optimise combustion conditions, less of the fuel's potential energy goes out unburned in the exhaust. Which also minimises emissions -- enough to fulfil with ease the US EPA's demand that 90 per cent of particulates and 80 per cent of nitrogen oxides are gone from diesel emissions by 2010.

It's been real-world tested, too. The converted heavy-duty Caterpillar diesel mill achieved a thermal efficiency of 53 per cent -- marginally better than the 50 per cent achieved by the most fuel-efficient diesel engine in the world.

Reitz's team claims the system's efficiency allows it to meet the most stringent emission regulations with room to spare, with fuel economy advantages such that if the system were rolled out into every petrol and diesel engine in the US, it would save the nation up to 4 million barrels of oil a day.

All that and it can be done using relatively cheap low-pressure petrol fuel injection technologies, rather than the costly high-pressure systems needed for high-compression diesel engines.

It's enough to have attracted further research funding from the Department of Energy.

Perhaps, too, it might flush Benz's DiesOtto out of hiding. Since debuting in 2007's F 700 Research Car, it's sunk below the horizon again.

DiesOtto shares some of the Reitz system's principles, shifting imperceptibly between compression and spark ignition with moment-to-moment changes in driving conditions and load. Low down it's a compression-ignited diesel, but kicking it down calls in the spark system and a squirt of petrol to grab some extra wattage when it's needed.

It, too, showed considerable promise on fuel efficiency, squeezing 175 kW and 400 Nm of torque out of just 1.8 litres and four cylinders while consuming just 5.3L/100km and emitting 127g/km of CO2. At the time they described the results as "the lowest petrol consumption figures of a large luxury saloon in the history of the motor car".

So where is it this apparent time of opportunity? Benz's local spokesman Peter Fadeyev says there's no news coming out of Stuttgart about it either way.

But that's not to say it's a goner, he adds. The company has a solid record in turning promising concepts into production realities. "Past history suggests it may yet reach series production."

An example? Fadeyev points to the F 200 research vehicle first shown at the 1996 Paris Motor Show. It spawned myriad technologies that rolled out in subsquent production models for years afterwards. Voice recognition for the phone in the 1996 W140 S-Class. Curtain airbags in the W210 E-Class of 1998. Active Body Control (ABC) in 1999's C215 CL-Class of 1999. Bi-xenon lamps in the W211 E-Class in 2003. The electro-transparent panoramic glass roof in the Maybach 62 in 2002. And the gullwing doors of the SLR McLaren in 2003.

"A lot more of that car's breakthroughs made it to market than didn't." Fadeyev says." So don't be surprised to see DiesOtto appear in the coming years."

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