Whereas up until now the automotive enthusiast knew of roughly only two large scale-use engines, the gasoline and the diesel, technological developments and new requirements imposed by economical or environmental reasons caused a type of "natural evolution" for the engine. It was only natural that at one point the battle between the low fuel consumption and emissions of a diesel and the performances of a gasoline would have ended, one way or another. But a marriage between the two... Mercedes Benz's DiesOtto engine promises to bring the best of both types of engines into one unit. By taking the "benefits of a diesel engine" and making it run on "on regular old unleaded", the German manufacturer envisions the future of the gasoline engine and perhaps the end of the diesel. The experimental 4 cylinder in-line 1.8l engine (238 horsepower and 400 Nm of torque), first showcased in 2007, is built on technologies, explained below, borrowed from both Rudolf Diesel's and Nicolaus Otto's creations. Homogeneous Charge Compression Ignition (HCCI)
Advantages of the system are that it provides up to 15 percent lower fuel consumption than regular engines. HCCI engines can work on diesel compression ratios, providing better efficiency than spark ignited gasoline engines while the even mix of fuel and air translates into lower emission.
The downsides are that high pressure in the cylinders may damage them in time, while high temperatures will wear down the engine faster than usual. The power range provided by HCCI engines is restricted and carbon monoxide and hydrocarbon emissions caused by the fast combustion are higher than in a spark ignited engine. Variable Valve Timing (VVT)
Usually, the position and shape of the cam profiles are optimized for a certain engine rpm, which limits low-end torque and high-end power. VVT allows the cam profile to change, resulting in greater efficiency and power, over a wider rpm range.Twin Variable Geometry Turbocharging (VGT)
VGT is a technology which allows the effective aspect ratio to change as the situation requires it. In terms we can all understand, VGT allows the turbocharger not to choke or fail to create boost due to engine speeds. By altering the geometry of the turbine as the rpm's increase, VGT maintains aspect ratios at the required levels for an optimum turbocharger response. VGT helps avoid pumping losses and lower power output. Variable Compression Ratio
The variable compression's disadvantage is that when operating under lighter loads, the system may cause the engine to lack power and torque. Nevertheless, the system is able to vary the inlet pressure and adjust the compression ratio in such a way that makes the unit be a small efficient engine that behaves exactly like a modern family car powerplant, yet it turns into a tuned one on demand.Direct Gasoline Injection (GDI)
In addition to the features mentioned above, the DiesOtto engine uses a starter alternator instead of a fly-wheel, a characteristic that makes it ideal for start-stop systems. The engine has two different operating modes, which make it act as both a gasoline and a diesel unit.
In low demand driving conditions, the engine uses the HCCI. When it needs to meet higher expectations, DiesOtto switches to a gasoline-like engine mode, using spark-plugs to ignite the fuel mixture. The two operating modes require different compression ratios, hence the need for a variable compression ratio system.
Even if Mercedes said at the time that the technology will probably become available sometimes in the future, no word on the engine has been heard of since. Still, for those who hope for a gasoline engine with diesel traits, the DiesOtto might not be over with, as Volkswagen is also developing its own "crossover" engine.