Mercury 175 HP Outboard Diesel Fuel Spark Ignition

Discussion in 'Propulsion' started by DCockey, Feb 2, 2022.

  1. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    I noticed that the Mercury 175 HP two-stroke outboard which runs on Diesel fuel with spark ignition is now available to commercial customers. It was originally developed for the US military. The engine uses spark ignition like gasoline engines, unlike conventional Diesel engines which have a higher compression ratio and ignite the fuel by the temperature of the compressed air in the cylinders.
    A two-stage direct-injection system uses a small charge of compressed air to finely atomize the diesel fuel as it injects it into the combustion chamber.
    Atomizing the fuel into a fine mist allows for ignition via spark to occur while delivering enhanced high rpm running quality.
    A glow plug works in concert with the spark plug for improved cold weather starting and idle quality.​
    Potential customers for the engine would be operators who want an outboard which uses Diesel fuel and not gasoline. I don't know of any other advantages over a conventional gasoline outboard.
    Diesel Outboard 3.0L Mercury Diesel Outboard | Mercury Marine https://www.mercurymarine.com/en/us/engines/outboard/diesel-outboard/diesel-outboard/

    Article about the engine when announced in 2015: https://www.boatingmag.com/mercury-racing-unveils-optimax-diesel-outboard/

    Evinrude has sold multi-fuel outboards to military only, but their engines were rated for Diesel only in emergencies.
    https://www.evinrude.com/en-US/multi-fuel/multi-fuel-engines-full-specs.htm
     
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  2. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    In other applications, diesel vehicles get around 30% more range compared to their gasoline equivalent.
    And since I'm currently looking at ethanol issues with fuel tanks, diesel would be very appealing.
     
  3. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    My guess is the Mercury spark ignition Diesel fuel outboard consumes about as much fuel than a contemporary gasoline outboard. Conventional compression ignition Diesel engines are more efficient than spark ignition gasoline engines for several reasons:

    The compression ratio of conventional compression ignition Diesel engines is usually between 15:1 and 21:1 while the compression ratio of modern spark ignition engines is typically between 9:1 and 11:1. Higher compression ratio is more efficient. I don't know the compression ratio of the Mercury engine but I guess it is similar to a gasoline engine or slightly lower.

    Diesel fuel produces more energy when burned compared to an equivalent volume of gasoline. The Mercury engine has this advantage.

    Gasoline engines have a throttle (variable restriction of intake air) which means at less than wide open throttle energy there are "pumping losses" because the pressure of the intake air is reduced (manifold vacuum). Conventional Diesel engines do not have throttles so there are no pumping losses. Spark ignition engines need to have a fuel to air mixture in a relatively narrow range so the amount of air in the cylinder needs to be reduced as well as the amount of fuel at lower power levels. Compression ignition engines can operate at very lean fuel to air ratios. My guess is the Mercury spark ignition Diesel engine has a throttle.​

    The 2015 article I linked to above describes the original military version of the engine as "spark ignited, direct fuel injected two-stroke" and "shares 95 percent of its components with the gasoline engine from which it derives." It appears to be derived from the no longer available 3.0 litre V6 Optimax and Two Stroke engines which had the same bore and stroke as the Diesel engine.

    Another guess is the Mercury Diesel engine is significantly more expensive than gasoline outboards of similar power.
     
  4. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Well, my enthusiasm has waned.
     

  5. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Well, that's Mercury's entire business model. Charge more, deliver less.
     
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