Mercedes-Benz’s gas and diesel duo

This article was originally on a blog post platform and may be missing photos, graphics or links. See About archive blog posts.

A quick auto-tech course: Gasoline engines get their power from igniting an air-fuel mixture with spark plugs, while diesel engines compress their air-fuel mixture to such a high degree that combustion occurs, um, spontaneously. Right? Gasoline has a more urgent feel under the right foot; diesel is known for being frugal.

So what if the best of both worlds could be combined in one engine? Mercedes-Benz is working on just that, using something called homogeneous charge compression ignition, or HCCI. Under low to medium engine loads, the diesel process is more efficient, so a gasoline-powered HCCI engine is able to adapt its compression characteristics to make the air-fuel mix (which can be leaner than usual) ignite by itself. Then, when more power is required, the compression ratio changes and the spark plugs kick in.

There’s a heck of a lot more to this. For example, the explosions in each cylinder have to be timed to the millisecond, and the timing for the spark-ignition mode is different from that of the compression-ignition mode. And how can the compression ratio be changed? By manipulating the crankshaft or the cylinder head? A whole college paper could be written on the challenges and advantages of HCCI.

Luckily, there are boffins doing all the homework and the driving public will enjoy the results, especially if they look as good as this F700, a full-size luxury sedan. It is equipped with what Mercedes-Benz calls a DiesOtto engine, honoring both Rudolf Diesel and Nikolaus Otto, whose name is given to the four-stroke cycle used in the vast majority of gasoline-powered engines. There’s also a 20-horsepower electric motor onboard, so the F700 is essentially a hybrid. But with a top speed of 120 mph and fuel consumption in the 40s, it’s an intriguing proposition. Too bad we’ll have to wait seven years or so to drive it -- or something like it.

-- Colin Ryan