Homogeneous charge combustion ignition makes for a new type of economical, low-emission combustion engine

There’s no need to reinvent the wheel, as it has proved its worth for 5,000 years now. But revamping the 131-year-old principle behind the gasoline engine is a different matter altogether. 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.

"Our goal is to make gasoline engines as economical as diesels, and the new DIESOTTO concept marks a major step in that direction, as it combines the best features of SI and diesel engines," says Thomas Weber, the Daimler Board of Management member responsible for Group Research and Mercedes-Benz Cars Development.

Along with low fuel consumption and thus lower CO2 emissions, such "best features" also include reduced nitrogen oxide (NOX) emissions. What’s more, the new drive system concept from the Daimler Engine Research labs in the Unter?türkheim ?district of Stuttgart, Germany, runs on normal gasoline, which means its modest fuel requirement can already be met at any filling station today.

The fuel/air mixture in the new engine is ignited by a spark plug (conventional SI method) during starting and full load operation, when the engine operates as a direct injection unit in the usual Lambda 1 mode. At low and medium speeds, however, the gasoline engine morphs into a self-igniting unit operating on the diesel principle. "Controlled auto ignition, which the DIESOTTO automatically switches over to within one power stroke, occurs under partial load conditions at low and medium engine speeds," explains Günter Karl from the Mercedes-Benz Untertürkheim plant’s Engine Research and Advanced Development department. As Karl, who played a key role in the invention of the new engine concept, explains: "This partial load state is the most common engine operation mode, which means it offers huge fuel-savings potential." The modular concept Karl and his colleagues developed for the new engine consists of the following elements: downsizing, turbocharging, gasoline direct injection, a variable valve train assembly and crankshaft drive, an innovative engine management and control system, and homogeneous charge combustion ignition — a new type of combustion process.

Engineers test the performance of a DIESOTTO unit on an Engine ? Research test stand. 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." By achieving this goal, the engineers laid the foundation for a new type of homogeneous charge combustion ignition that required basic changes to be made to power strokes and the combustion process. The result is a gasoline engine that self-ignites rather than being ignited by a sparkplug. "Our homogeneous charge combustion ignition marks the start of a revolution in terms of the exhaust stroke because the DIESOTTO does not completely discharge its exhaust gas," says Karl. 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. Homogeneous combustion 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. Experts therefore refer to this as "lean operation." 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," Karl’s colleague Rüdiger Herweg explains. "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. Less nitrogen oxides 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 — much less than with the "lean concepts" used in the past. Conventional combustion with sparkplugs, on the other hand, can generate peak temperatures and hot spots, both of which facilitate the formation of nitrogen oxides. "Because this doesn’t occur with homogeneous charge combustion ignition, all the DIESOTTO engine needs for exhaust treatment is a standard three-way catalytic converter," says Daimler’s Chief Environmental Officer Herbert Kohler, who is also head of the Body and Powertrain Research Directorate at Group Research and Advanced Development. 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. Efficient control Combining the various individual technologies into a practical and effective drive system required the utilization of a sophisticated concept, which in this case took the form of an extremely efficient engine management and control system developed by Daimler engineers. Whereas conventional engine control systems tend to be relatively "stubborn" units that work through pre-programmed algorithms, the control system used with the DIESOTTO also incorporates a closed control loop that uses the combustion chamber sensors to monitor and actively control the engine online. Any doubts about whether a self-igniting gasoline engine would function properly have been put to rest by the Mercedes-Benz F 700 research vehicle, whose 1.8-liter DIESOTTO engine delivers 175 kW (238 hp) while consuming only 5.3 liters of fuel per 100 kilometers and emitting just 127 grams of CO2 per kilometer. The vehicle therefore already meets the future EU 6 emission standard and demonstrates that despite its more than 100-year history, the internal combustion engine remains young and innovative.