While we’re not usually the types to get excited about outboard boat engines, this one is something to behold. Called the Mercury Racing OptiMax, it is a 175hp, 3.0L V6 diesel engine that was built using 95% of the same parts used in the company’s popular gas outboards, and the darndest thing? It fires the diesel fuel off with a spark plug, not be the traditional compression ignition of the diesels we know and love in trucks, tractors and other places. The engine was developed by Mercury for the Department of Defense after they came down with a mandate about five years ago that all gasoline burning engines needed to be banished from the boats and ships that they operated. Fires and damage from said fires apparently sparked this move and now Mercury makes an outboard that runs on JP fuel and has added this diesel to the mix. For the time being it does not seem that these will be made available to the public. We’re going to keep an eye on that.
So how does the engine manage to fire the diesel fuel with spark plugs? Air is a decidedly big factor in this. As the fuel charge is coming in, high pressure air is blasted in as well and that helps to atomize the diesel to the point that a normal spark plug will ignite it. A glow plug is used on startup and during idle to maintain proper combustion while temps are lower in the head apparently.
Obviously the company has done lots of testing on this engine and its components, but is anyone else freaked by the idea of having a diesel engine that is effectively identical to the gas variants? In the automotive realm, this strategy has not worked for most everyone that tried it. Given that this whole package is far more advanced, maybe there is nothing to worry about but (we’re looking at you Oldsmobile) there are some lingering doubts.
The engine is a two stroke design and while the weight was not given we’re sure it is damned close to the gasoline burning 3.0L variant as the previously mentioned parts sharing should make it so. That is unless the block is one of the 5% of parts NOT shared by the other engines. We are very interested to see and hear this engine run. We bet it has a weirdo exhaust note and we’re also curious if it would qualify as a diesel engine by the rules of land speed racing. It does run on diesel, just not on the traditional way that we’re used to.
CLICK HERE TO READ THE FULL STORY ON THE OPTIMAX DIESEL OUTBOARD
Huh. Neat!
I’d like to see this in action.
Diesel does not burn as well as gasoline…unless… a high pressure injector line ruptures and sprays it on a turbo that many marine engines use these days. Seen it many times.
First of all, thanks for the heads-up on this.
As far as LSR – you would have to check with Jim Dunn – he’s the SCTA truck committee chair and he’d poll the committee for a ruling. I suspect anyone asking to run this engine would cause a rule change. Current wording just says “Diesel engine” and I suspect it might get changed to “compression ignition and fueled with Diesel fuel” (or something like that) leaving out an engine running on Diesel fuel but requiring spark plugs to ignite. I think I’d be OK running against this engine but it really doesn’t work on the principals of the Diesel cycle.
Yeah – I got a horse in this race.
Dan
Email sent to Mr. Dunn. I’ll post his take on it.
Dan
I thought the whole idea of Diesel power was to lose all the electrics and make the engine waterproof – which really helps in a marine engine!
That’s why 99% of ships have Diesel engines – durrr!
In this case, it seems the military just doesn’t want to deal with the logistics hauling around a bunch of different fuels and wanted to fuel everything from the same supply.
Very true. The logistics tail is already too long. All vehicles are moving to JP8 or JP4. It is rough on diesel engines though because it is a dry fuel.
Interesting stats:
2 stroke, 175hp
Max engine speed of 6000rpm
“direct injection” at 95 psi.
Implies fuel is injected as soon as the exhaust port is covered. Definitely Otto cycle, not Diesel cycle.
I checked with the folks at SCTA. IF (and that’s a big IF) it can run without the plug wires connected and on our event Diesel fuel (D2) it’s not a Diesel and would run in one of the fuel classes depending on body class. So if, for example, they ran it with spark plugs to warm it up and them disconnected them for the actual run, it might be classified as a Diesel – doesn’t sound like that’s how this thing works.
The concept is interesting to Jim and I’m sure others at SCTA and they’re kicking it around. I’ll let you know if there’s any change in their thinking.
Dan
Forgot to mention – As I understand it there is no requirement that Diesels be direct injection. Some older compression ignition engines were not DI though I don’t think they worked very well (power/displacement). I’ve heard the argument that my Mercedes OM617 is IDI because the injection is into a prechamber rather than directly into the combustion chamber. I’m not buying that argument as it still relies on timed injection to make it work.
Dan
Hey Brian,
When you were at the Farmall Museum, they should have had some diesels that started on gas and ran on diesel. A lot of Farmall diesels had both spark plugs and injectors…they weren’t firing the diesel with spark plugs, but many of the components were the same from gas to diesel.
We had a Farmall M diesel on our farm, when I was a kid and it started on gas. They are cool engines.