The Differences Between Diesel and Gas Marine Engines

There are several differences between diesel and gas engines on boats. Both types have their advantages and disadvantages, and we will be going over some of those in this post. We will be examining some of the basic differences in the fuel, as well as how the basic function of the engines is different as a result. We will also be going over the practical applications of both diesel and gas engines in boating today.

Differences in How Combustion Occurs

Both types of engines require three elements to properly combust, but they approach these elements differently. All combustion requires fuel, oxygen, and heat to burn. While the fuel is different for these they act in the same purpose for both types of engines. The oxygen in the air acts the same, even if diesel needs more oxygen than comparable sized gas engines. The biggest difference between gas and diesel engines is how they produce the heat needed to start combustion. Gas engines use spark plugs to arc electrical energy across a small space in the combustion chamber of the engine. Gas engines do rely of some compression to heat the injected fuel as well. Diesel engines on the other hand almost exclusively rely on compression to generate the heat needed for combustion. To get that heat diesels usually have about twice as much pressure as a gas engine.

How Diesel and Gas Fuels are Different

Gas fuels are significantly more volatile than diesel fuel. That means they are less stable and are more easily ignited than diesel fuel. They burn faster as well. But burning faster does not mean that it burns more efficiently from a power producing perspective. One of the problems with a more volatile fuel is that it evaporates significantly more quickly than diesel fuel and cannot be stored as long. This can be a serious problem in some marine applications.

Diesel fuel is significantly more stable than gas. This is great for storage but does create a challenge of how to get it to ignite. Diesel fuel requires proper atomization and high levels of compression to properly combust. Diesel engines are generally very heavy duty and are built to handle being run for a long time at high loads. As a result, they tend to be reliable as long as they are properly maintained.

Practical Applications of Gas Engines

Gasoline powered engines have several applications where they are beneficial. They are great their reliability and serviceability, which make them an asset for fishing and pleasure craft. The relative weight of outboard engines compared to comparable sized diesel engines give them an advantage on lighter weight vessels, like center consoles or flats boats. Smaller boats get a real advantage when it comes to outboard gas powered engines. Limited space within the boat makes inboard diesel engine impractical.

Diesel Engines and When They are Most Beneficial

One of the biggest benefits to diesel powered boats is the high torque the engines provide at lower RPMs. That makes diesel king on larger vessels, or vessels that operate at their high range rpms for longer periods of time. The high torque allows diesels to push large vessels through the water, but at generally lower speeds than a gas powered engine. That makes diesel perfect for work boats or large boats with a lot of displacement. Diesel engines are also great for generators, which are more common on larger vessels, especially those that sit at anchor so the boat engines do not have to be run to power accessories on the boat. Sailboats also frequently have a small diesel engine to help with docking. The displacement hull makes the diesel’s high torque perfect, plus the less volatile fuel can keep for longer when the boat is primarily using wind for propulsion.