Leaving a Diesel Running

Some people say you should leave a diesel engine running. With modern diesel-fired boats, though, shutting the diesel engine down when you don't need it to move the boat may be a better alternative, whether you're shutting down for just for the few minutes it takes to clear Customs or for a few days of shopping. It's a question of saving fuel and maintenance costs.

  1. Marine Fuel Consumption is Different

    • Marine fuel consumption isn't based on fuel consumption per mile, but on fuel consumption per hour. This means that for every hour you leave the vessel's engine running, the fuel consumption continues. If you leave the engine running for a single minute, the fuel consumption is still continuing. If you shut the engine off, the "gallons-consumed-per-hour" clock stops with it.

    The Theory Behind Letting the Diesel Idle

    • If you've been around diesel engines, you've no doubt heard the old-timers say that it takes "more fuel to start it than it burns at idle." A diesel engine doesn't have spark plugs. The heat for combustion comes from compressing the air and fuel mixture. At idle, the temperature drops inside the engine's cylinders. Combustion isn't complete and the engine runs less efficiently. Translated, this means it burns more fuel at idle than it does at "dead ahead."

    The Problem With the "Let It Run" Theory

    • At idle, the NTA-855D will consume about 28 percent of the fuel it would consume at its maximum speed. For the NTA-855D, that's 48.6 gallons per hour at full speed and 9.8 gallons per hour at dead slow, which translates to about 21 ounces per minute. Modern marine diesels are equipped with high-efficiency air or electric starting systems that require less than five seconds to initiate movement, compression and the resulting combustion in all of the engine's chambers, as long as there's fuel and the engine is in good working order.

    Let It Run Equals Let It Burn More Than Fuel

    • A diesel engine that's allowed to idle excessively will cost you in substantial maintenance problems, as well as the money for the fuel needlessly consumed at idle. Since lower cylinder temperature at idle doesn't result in complete combustion, you'll find carbon in the oil, on the valves and on top of the piston. You'll find carbon buildup within the cylinders. The engine's air box will appear to leak oil, carbon specks will fly out of the exhaust ports. The fire rings on the piston and the oil control rings will be coated in carbon and your engine's efficiency at all speeds will suffer.