The Miller Engine

Authored by: Wayne G. Slife, May 2004

Brought to you by, comments to

The "Miller engine" was a development of the Danish (resident of the US) inventor, Ralph Miller. It was a form of variable valve timing that increased the BMEP (Brake Mean Effective Pressure) of supercharged 4 cycle diesel engines from 120 to 160 pounds. An increase of 33% and with reduced firing pressures and peak bearing pressures, astounding thermal efficiency, low lube oil consumption, etc. 

It had similar advantages of a spark-fired natural gas engines, plus, it successfully addressed the problem those engines had with poor scavenging, and increased fuel consumption at reduced loads. The Miller Engine was licensed to Nordberg Manufacturing Co of Milwaukee, Wisconsin, for engines 9" bore and larger. 

Nordberg made its money with big, 2 cycle units, 21.5x31 (which had marine applications) and 29x40, but these were standard open chamber, port-scavenged types, not Miller engines. All of my work for Nordberg was in stationary engines (power generation, pumping stations - liquid and gaseous, etc.). and our main propulsion drives were of that design. Having 13" bore and 16.5" stroke, in 6 and 8 cylinder in-line, as well as 12 and 16 cylinder V type configuration. I am not sure if they made a marine version, but Nordberg's trade name for their Miller engine was "Supairthermal" engine. 

Here is a brief, if not inadequate, description of the Miller cycle:

As you know, a straightforward atmospheric two-cycle, fuel oil, diesel engine develops more power than an atmospheric four-cycle unit of approximately the same size, thus making it cheaper per horsepower to manufacture. After all, every down stroke is a power stroke, vs. every other down stroke in the four cycle - or, as the four cycle makers rosily put it, the engine "rested" or "cooled off" every other stroke. Even allowing for some limitations imposed by much greater heat being developed in the two-cycle engine (especially troublesome as displacement went down and speed went up), the atmospheric four cycle makers were at a cost and selling price disadvantage.

Around 1940 The Elliott Company, of Jeanette, Pennsylvania began to manufacture, under license, the exhaust turbocharger developed by the Swiss firm, Buchi. It was the answer to a maiden's prayer for four-cycle engines, increasing Brake Mean Effective Pressure (BMEP) from 80 psi to 120 psi with little adverse effect. As HP is directly related to BMEP that meant 50% more horsepower from the same displacement, thus redressing the cost disadvantage. And, if that weren't enough, it was so easy to accomplish that old four-cycle units could also be retrofitted with the Elliott unit and derive the benefits. Admittedly one had to use common sense here - increasing horsepower by 50% on an early diesel engine carried some risk. It might be vulgarly compared to prescribing Viagra to a hopeful centenarian.

Another 10% might be squeezed out by installing an intercooler between the turbocharger output and the engine intake valves, thus raising BMEP to 132 psi. There it rested until Ralph Miller came along with his design and increased the BMEP to 160 psi. 

Obviously just increasing turbo output and sending greater quantities of intake air (increasing intake air pressure) to the engine was not feasible. During the compression stroke that charge reached excessive pressure and, following Boyle's Law, heated excessively. These elevated values for pressure and heat would be fatal. Miller's insight was, like all really great insights, simple. He still used an intercooler, and still used the new high pressure turbo output, but he reduced the pressure and the temperature of the air charge by closing the intake valves before bottom dead center, thus bringing pressures down to normal values, and chilling the air to normal temperature as well. He was, in effect, doing outside the combustion chamber some of what other engines did inside.

I mentioned that Ralph's insight was simple. It seems that those are the ones most difficult for "experts" to get their heads around. I had a number of such personal experiences with learned academicians and engineers, including some of them so carried away by the "impossibility" of our claims to reduce the pressure and temperature in the combustion chamber by expanding the air charge, that they said that Boyle's Law didn't apply in the short time (hundredths of a second) between valve closure and bottom dead center. Only later, when these experts had cooled off, did they admit that what they had said was, how shall I say it, regrettably excessive. They were still damned doubtful, however, regardless of what Boyle said.

The valve closure of the Miller engine was governed by a link shaft arrangement, and varied automatically with engine load. Turbocharged spark-ignition gas engines were also fitted with the Miller system, but intake valve closure was retarded until after bottom dead center for good and sufficient reasons beyond the scope of this brief introduction.

As you can perhaps tell, I had a hell of a lot of fun in the diesel engine business, more fun in fact than anything else I've ever done in a full and largely mis-spent life. Mazda has offered an automotive version, although the original Miller engine featured early inlet valve closure, which the Mazda incarnation did not.

Mr Slife was a Sales Engineer for Nordberg and spent countless hours talking with Ralph Miller about his fascinating Miller cycle. Mr. Miller had an off-putting no-nonsense flavour to him, but when one got to know him well, he made great sense.
Wayne G. Slife, 8240 Pompano Street, Navarre, FL 32566 (update 09.2009 - this address may no longer be valid - ml)
Brought to you by, comments to