Marine Diesel Engine Maintenance

A naturally aspirated (NA) engine is one in which air is drawn into a cylinder by the movement of a piston down the cylinder. This movement lowers the pressure in the cylinder and ‘sucks’ in air. A turbocharged engine has a device that pumps in air, enabling substantially greater amounts of oxygen to be introduced. This allows more fuel to be burned, which increases the power from that engine. This is reflected in an increased power-to-weight ratio, which is often published. 

Crudely speaking, a turbocharger has two fans on a common shaft. One fan (the turbine wheel) is set in the path of the exhaust gases as they exit the engine, and the other (the compressor wheel) is in the path of the inlet air. When an engine is running, the exhaust gases, which are moving at a high velocity and still contain a significant amount of unexploited energy, spin the turbine wheel. This spins the compressor wheel, forcing air into and pressurizing the air into the engine’s inlet manifold.

 A turbocharger is, in effect, an air compressor driven by the exhaust gasses. Whenever an intake valve opens, this pressurized air is driven into the cylinder as opposed to being drawn in by the piston moving down the cylinder.

At high engine loads and speeds, turbochargers can create excessive inlet (boost) pressure. Many have a wastegate, a device that responds to this high pressure by opening a bypass valve on the exhaust side, reducing the gas flow over the turbine wheel. Turbochargers on many modern engines are considerably more complex, with moveable vanes (variable geometry turbos, or VGT). All can spin at extraordinarily high speeds from 100,000 rpm up to 400,000 rpm in the latest generation devices. Some larger engines have more than one turbocharger with the turbochargers installed in series. 

Intercoolers and aftercoolers

When air is pressurized, what happens? The temperature goes up! You might think this is a good thing because we want high temperatures to ignite the diesel fuel, but hot air at a given pressure has less density than cooler air, and less density means less oxygen. For example, air at 50˚C (112˚F) contains 14% less oxygen than air at 7˚C (44˚F). So typically, immediately after a turbocharger, there is a device to cool the air – known as an aftercooler or intercooler (the two terms tend to get used interchangeably, although with the typical single-stage turbocharger, 'aftercooler' is technically more correct, but we have employed the more commonly-used 'intercooler'). This compressed cool air then enters a cylinder where the pressure is raised high enough during the compression stroke for ignition to take place. If you have a turbocharger, you likely (but not necessarily) have an intercooler. Intercoolers require maintenance. We will return to them.

By driving more air into a cylinder of a given size, a turbocharger raises the effective compression ratio. This enables the actual compression ratio - what it would be without the turbocharger - to be lowered to levels that are just adequate for starting. At startup, there is no turbocharger boost pressure, so there is a limit to how low the compression ratio can be. This is generally around 16:1 or 17:1.

Rated Life Expectancy

With or without a turbocharger, given a recreational duty cycle, most of the small marine diesel engines installed in recreational boats have a life expectancy rating before a major overhaul of 5,000 hours. This duty cycle assumes that operation at wide open throttle (WOT) is intermittent. These engines are not rated for continuous duty at WOT.

Instead of hours, the life expectancy before a major overhaul of commercial engines is sometimes rated in the volume or weight of fuel that will be burned. The more power that is extracted from a given engine the faster the fuel will be burned and the less the operating hours before the overhaul.

>>>>>

Nigel and Jan's course has 86 lessons similar to this with handy "how to fix" instructions for each engine component. Get the course now.