It seems that Mercury Racing can get an OptiMax two-stroke outboard to run on any combustible liquid. The latest project to come out of “The Ledge” is a 3.0-liter Mercury OptiMax outboard engineered to run on ultra-low sulfur diesel fuel. The project is a response to a continuing initiative by the United States Department of Defense to achieve a single “battle space fuel,” so that all vehicles can operate on a single fuel type, and to eliminate gasoline fuel for safety reasons whenever possible.

mercury optimax

Intended for defense duty, the Mercury Racing OptiMax Diesel features low-key black-on-black graphics.

Six years ago we reported on the Mercury Racing OptiMax JP, a 3.0-liter outboard that runs on JP5 jet fuel. That motor has been operational for some time, according to Mercury Racing marketing manager Steve Miller, and the company is presently building those engines for DOD customers. But worldwide, diesel fuel is ubiquitous in the public supply and in use at most military facilities, so now the goal is to make diesel the battle space fuel when possible.

Converting an OptiMax outboard to run on diesel was “like getting it to run on 40-octane gasoline,” said Steve Wynveen, part of the team of engineers and technicians who worked on the project for the past three years. The outboard is unusual for a diesel in that it’s a spark-ignition engine – the fuel charge is ignited by a spark plug as on a gasoline engine – rather than a compression-ignition engine as is typically the case in diesel design. The feature that makes this possible is the OptiMax two-stage direct-injection system, which utilizes a burst of compressed air to inject and atomize the fuel charge into the combustion chamber. That pressure was increased from 80 to 95 psi, and Mercury was able to get diesel fuel atomized into a fine enough mist, droplets of 10 to 20 microns, to enable spark ignition, with some significant redesign of the combustion chamber and port timing. Then engine always operates in stratified mode, with the plume of fuel contained in an area close to the spark plug and surrounded by mostly air. A standard OptiMax operates in stratified mode at idle and low speeds, but then fills the engine combustion chamber with a homogeneous fuel mixture at higher engine speeds.

“The diesel engine never has a homogeneous mixture,” said Wynveen. “We are actually using an earlier version of the OptiMax injector on this engine, which produces a wider cone angle of the injected fuel/air mixture. We also relocated the spark plug in the cylinder head, and it’s recessed about a quarter of an inch further away from the chamber compared to the gasoline engine.”

spark ignited diesel

This CAD image of a Mercury Racing Diesel combustion chamber shows the position of the glow plug (purple) and spark plug. The central OptiMax air-assisted direct injection unit creates a cone of fuel vapor (pink) that is wider than the cone created by the injector used in current gas-powered OptiMax outboards. The diesel cylinder head has more of a dome shape, and the "crater" in the piston dome has been re-shaped to better manage the plume of fuel vapor during stratified combustion.

The OptiMax Diesel is also equipped with a glow plug, which heats the combustion chamber and helps vaporize the fuel to a combustible state when the engine is cold-started. The motor is otherwise standard-issue OptiMax. Mercury Racing says the OptiMax Diesel uses 95 percent of the parts found on an OptiMax 300SX outboard. Output for the OptiMax Diesel is 175 horsepower, with a WOT range of 5000 to 6000 rpm. The motor uses a Fleet Master gearcase with a 2:1 gear ratio. While this engine is not intended to have real “stealth” capabilities, Mercury did modify the cowl and exhaust bucket to reduce sound output.

“This motor will typically see harbor patrol duty with the Navy,” said Miller. “The usual application is an 8.5-meter RIB with twin outboards, which may be onboard and deployed when a ship is docked at a foreign port, for example.”

Miller adds that Mercury has received a number of unexpected inquiries about purchasing the OptiMax diesel for civilian use, but there are no plans to make it available in the near future.

“We did not design this motor to meet emissions regulations, although that would not be too challenging,” said Miller. “But that would be a step we’d have to take before we could offer it to the public.”

According to Mercury the OptiMax Diesel is presently undergoing a final round of validation by the Navy.

For more information visit Mercury Racing. Another unusual engine that may be of interest is the Evinrude 55 MFE multi-fuel outboard.