No knocks or NOx

The first truck I ever drove (in Vietnam in 1968) was an Army two-and-a-half-ton cargo vehicle powered by a multifuel diesel engine. It sounded more like a car engine than a truck diesel and would burn anything but high-octane gasoline. Even more interesting was that even though the engine used compression ignition, it didn’t require fuel with a high cetane rating.

In 1973, I interviewed Max Fiedler, a local engineer who wanted to produce a small diesel engine he said was very light, would run more quietly than a standard diesel engine and would do so without smoke. Fiedler’s engine defied all the standard diesel rules about how the injection system should work. He used very low injection pressures – 1,200 psi or less.

Today’s diesels use 26,000 to 30,000 psi. Normal diesels use the timing of the fuel injection to control the timing of the combustion. I thought the engine might smoke or knock due to poor atomization, but a demonstration dispelled that concern. The little machine started and ran smoothly with no diesel knock or smoke – white or black.

Fiedler bolstered my confidence in him by presenting me with a copy of his 1939 paper, published by Philadelphia’s respected Franklin Institute. I have since read extensively about advanced diesel combustion systems. I discovered, for example, that Sigfried Meurer, the German engineer who masterminded the engine used in my Army truck, agreed with Fiedler on most of the basic points of his theory.

The theory is quite simple: If you can thoroughly mix diesel fuel with air, you will prevent combustion knock and diesel smoke. Reducing audible knock also reduces nitrogen oxide emissions.

Few engineers today discuss Fiedler or Meurer, but there is a rapidly growing body of research based on their work. It’s called HCCI, or “homogeneous charge, compression ignition,” sometimes called “pre-mix” diesel combustion.

Beginning in the early 1990s, Japanese engineers picked up the idea – or perhaps even reinvented it. In 1999, a group of engineers from Nissan Motor Co, Ltd. and the Musashi Institute of Technology published a paper on the MK engine – a small, direct-injection, turbocharged car engine. The engine uses cooled EGR – as much as 32 percent of the volume of gases taken in through the valves is recycled exhaust – and its injection system has some basic similarities to Fiedler’s.

In a standard diesel engine, fuel burns as it is injected after a brief initial delay. In the MK engine, fuel is injected in a very short spurt, and ignition is delayed until some time later. The fuel has 20 to 30 degrees of crankshaft rotation to mix with before it ignites. The result is an engine that is smokeless in spite of the high rate of exhaust gas recirculation. It also runs with almost no diesel knock and reportedly meets modern car emissions standards without a catalytic converter – something a diesel has never done before. Some day soon, this totally different type of direct-injection diesel will probably appear in a Nissan passenger car.

It is conceivable that this kind of combustion system could be adapted to a truck diesel engine. One barrier is the fact that truck diesels produce more torque than any other kind of internal combustion engine, and the standard diesel process lends itself especially well to doing this without causing explosive combustion. Most pre-mixed diesel combustion systems are limited in their torque-producing ability. But if they could be made to produce enough torque, they might enable a driver-friendly truck diesel engine to meet emissions standards with a minimal reliance on aftertreatment.