Diesels operate by compression to ignite the fuel, not by spark plug as gasoline engines do. This means that the interior temperature of the cylinder needs to reach about 750 degrees Fahrenheit at full compression to start and 1,000 degrees Fahrenheit before the engine will run efficiently.
For example, if the temperature outside is 100 degrees and you compress the air at a ratio of 10 to 1, then the air in the cylinder will be 1,000 degrees at full compression. Most diesels have a compression ratio of 17:1 or better, in order to achieve this temperature using ambient air of about 45 degrees (17 x 45 = 765).
The math is important. With a compression ration of 17 to 1, the engine simply can't start when the temperature is below 45F.
But the typical diesel engine gets a little help.
On engines with compression ratios of 17:1 glow plugs are provided to add about 60 degrees to the interior of the cylinder prior to cranking, which helps the cylinders reach 750 degrees during compression on those cold mornings.
In my experience, smaller engines that are not fitted with glow plugs will have a compression ratio of about 20:1 to achieve the same interior temperatures, but that 20:1 compression requires a stronger battery and clean electrical terminals to achieve that initial start.
Operating on warm days is different. If it is 90 degrees outside, almost any diesel will start right up without using the glow plugs. But diesels never run efficiently until the cylinder temperature reaches 1,000 degrees, which usually takes about 20 minutes of running under load or 30 minutes at the dock without it. Suddenly, all the sluggishness is gone.
As a commercial charter captain, standard operating procedure is to start the diesels at least 30 minutes prior to departure. This gives time to figure out if anything is wrong with the engines and gives the engines time come up to full operating temperature, allowing full operational capabilities to accomplish any maneuver required underway.
Next time you charter, give a listen as you walk on board. The engines are almost always running before the passengers show up.
Capt. Alan Hugenot is a naval architect and marine surveyor based in San Francisco,whose writing has appeared regularly in Sea Magazine, Latitude 38, The Log newspaper, 48 Degrees North, Go Boating and many other boating publications on the Pacific coast. He serves as National Chairman of the Motor Yacht Technical Committee for the Society of Naval Architects and Marine Engineers.