Mercedes Benz DIESOTTO engine explained

The following has been extracted from a Daimler press document. I’ve taken out all the media speak in an attempt to provide an understanding of what the DIESOTTO engine is and how it works.

With the new DIESOTTO, Mercedes-Benz has launched a completely new engine concept in which a gasoline engine can temporarily function like a self-igniting diesel.

Performance from a 1.8L DIESOTTO engine (as fitted to the F700 concept car)

Power: 175 kW
Max. torque: 400 Nm
Acceleration: 0-100 km/h in 7.5 sec
Consumption: 5.3 l/100 km
CO2 emissions: 127 g/km
Emissions rating: EU 6
Exhaust gas treatment: Catalytic converter

The fuel/air mixture in the new engine is ignited by a spark plug during starting and full load operation. At low and medium speeds, however, the gasoline engine morphs into a self-igniting unit operating on the diesel principle.

This partial load state is the most common engine operation mode, which means it offers huge fuel-savings potential.

The first step toward creating the fuel-efficient DIESOTTO involved downsizing from a large volume, six-cylinder naturally aspirated engine to a compact four-cylinder unit. The resulting loss of output was more than offset by a two-stage turbocharger system that has a large low-pressure turbocharger connected to a high-pressure turbocharger with a smaller diameter. Together, they cover the whole range of engine speeds.

Gasoline direct injection is another key component of the DIESOTTO technology package.

Whereas direct injection in current production engines like the 350 CGI is geared toward stratifying an ignitable air-fuel cloud around the sparkplug, our goal with the DIESOTTO was to employ direct injection mainly to create a good homogeneous mixture and control the auto ignition function.

The DIESOTTO does not completely discharge its exhaust gas. Instead, the exhaust valves open for only a very short time and with a small lift, which is why only half of the exhaust gas volume can escape from the cylinder. The remaining 50 percent stays in the combustion chamber, along with most of the exhaust gas heat. To increase the temperature even further, a small amount of fuel is also fed into the cylinder by the injectors. The fuel doesn’t ignite, but it does thermally react with the hot residual gas.

During the subsequent intake stroke, fuel is injected into each cylinder in individually dosed amounts. The intake valve then opens briefly and with a small lift, at which point a precisely measured volume of fresh air flows in and mixes together with the residual gas.

During the subsequent compression stroke, the temperature of the mixture of residual gas, fuel and air continues to rise. The air-fuel ratio is set at such a level that more air is contained in the cylinder than is necessary to ensure complete combustion of the fuel. The next power stroke commences when the temperature has risen to a level that triggers controlled auto ignition. The coup Daimler engineers have achieved here lies in their ability to determine the exact time of combustion through manipulation of the fuel volume and the injection point.

We get the data for this point from combustion chamber pressure sensors in each of the four cylinders. Because the mixture ignites at many places simultaneously, combustion is extremely even, or homogeneous.

This offers two environmental benefits: namely, lower fuel consumption and reduced nitrogen oxide emissions.

Fuel efficiency is primarily enhanced by a lean fuel-air mixture with a high proportion of exhaust gases and a compression ratio that is adjusted in line with load conditions. Because homogeneous combustion is an even process occurring at relatively low temperatures, only an extremely small amount of nitrogen oxide is produced. All the DIESOTTO engine needs for exhaust treatment is a standard three-way catalytic converter.

Another key element of the DIESOTTO engine is its variable valve control system, which enables both the valve opening intervals and the lift to be adjusted. To this end, the camshafts have two different-sized cams for each valve, with the larger lift used at high engine speeds and loads and the smaller one employed mainly for homogeneous charge combustion ignition in partial load states. The DIESOTTO is also equipped with a variable crankshaft drive to ensure that it can meet the differing demands of partial and full-load operations. This unit makes it possible to variably adjust the compression for each piston, which gives the engine a variable — rather than a constant — compression ratio.

 

Source: Daimler

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