Diesel Engine Basics and Core Systems Identification

A diesel engine is a fundamentally reliable piece of equipment. More so than a gasoline engine. Nevertheless, over the decades we have seen plenty of engine failures in recreational boats, but what we have never seen is a diesel engine that has worn out. This is different to industrial and commercial applications.

In recreational boats, engines fail from inappropriate duty cycles (primarily, long hours at light loads) combined with inadequate maintenance and neglect. Electrical issues will prevent an engine from starting. Fuel related issues will cause it to quit. Rust and corrosion will attack it peripherally. 

Although dirty oil, or a lack of oil, and overheating can result in instantaneous catastrophic damage, most failures develop over time. There is generally an opportunity to head them off. If this is not done, at best engine failures are an expensive inconvenience; at worst they threaten the safety of the crew and boat.

These failures are almost all avoidable. If an engine is properly installed and maintained, and subjected to an appropriate duty cycle, it will run anywhere from 5,000 to 15,000 hours without requiring anything beyond routine maintenance, and certainly no significant overhaul. 

In this module we are going to cover all those aspects of owning a diesel engine that, if met, will ensure it provides a long life of troublefree service. In the event problems do arise, we will cover techniques which will enable most to be identified and resolved.

The goal is to take the anxiety out of owning a diesel engine so that you can enjoy a stress-free time on the water.

No ignition system

We will first dive into a little theory. An understanding of the core components and basic operating mechanisms of a diesel engine will help you understand why the maintenance we will cover is so important. It will also come in handy when we get to troubleshooting.

Unlike a gasoline engine, a diesel engine has no ignition system. This raises the question as to how we are going to get the diesel fuel to burn, which is an essential precondition for engine operation. As unlikely as this sounds to many people, it is done by compressing air. Maybe as a kid you remember pumping up a bicycle tire. The pump got warm. This is because there is some heat contained in all air, no matter how cold the ambient temperature. When you compress air, you concentrate the heat, and the temperature goes up. At high enough levels of compression, the temperature rises above the ignition temperature of diesel fuel – any diesel added to this mass of highly compressed, super-heated air will ‘spontaneously’ combust without any additional ignition source.

A diesel engine operates by compressing air to a level at which, no matter how cold the incoming air may be, the resulting temperature is high enough to ignite diesel fuel. This compression is achieved by trapping air in a cylinder and then forcing a piston up the cylinder to compress the air.

It has been found that if air is compressed to one fourteenth of its original volume – this is known as a compression ratio of fourteen-to-one, or 14:1 – the compressed air will almost always be hot enough for ignition to occur. Higher compression ratios – some diesels go as high as 22:1 – provide an added margin for really cold conditions, and for wear and tear which reduces compression. 

At the lower compression ratios, to ensure the compressed air reaches ignition temperatures no matter how cold the climate, it is common to have some kind of a heating element (typically, a glow plug) which is activated prior to cranking an engine. It preheats some of the air before it is compressed, ensuring ignition temperatures when the air is compressed. As soon as the engine fires up, heat from combustion warms the incoming air and the pre-heater is no longer required. 

If you have to hold your ignition key against spring pressure, or push in a spring-loaded button, for ten seconds or so before cranking your engine, you have glow plugs (or some other pre-heat device).

The ratio between volume A and B is the compression ratio

The relationships between compression ratio and pressure and temperature

In the U.S. there are two diesel fuel formulations that are widely available - #1 and #2 – with slightly different ignition temperatures. Number one ignites at 210˚C/410˚F; number 2 ignites at 257˚C/494˚F. 

Number 2 diesel is found in all warm weather states year-round and most cold weather states in the summertime. Number 1 is a cold weather formulation which is found in northern U.S. states in the wintertime. It prevents the congealing of fuel-system-blocking waxes that occurs with #2 diesel in extremely cold temperatures. Except for those intending to boat in seriously sub-freezing temperatures, both #1 and #2 diesel are fine in our recreational boats.

In Europe there is a similar differentiation with "winter diesel"  sold in northern countries, usually from November until February.

Ensuring full combustion

As soon as the diesel fuel combusts, the temperature in the cylinder rises. Because pressure is a function of temperature, in the same way temperature is a function of pressure, the pressure goes up. It is this increase in pressure which is used to power an engine. 

For an engine to run, the moment of combustion must be coordinated with the movement of pistons and other components. If the diesel fuel were to enter an engine mixed in with the air supply, as is the case with the fuel in many older gasoline engines, the diesel would ignite somewhat randomly as soon as the air under compression reached the diesel’s ignition temperature. The engine would never run. The diesel fuel must be introduced at a precisely controlled moment in the engine’s operating cycle. This is the function of a fuel injection system. 

The burning of the diesel fuel is a chemical reaction between molecules of hydrocarbon in the diesel fuel and molecules of oxygen in the compressed air, which means the molecules must first find one another. For complete combustion to occur, every molecule of diesel fuel must find a molecule of oxygen. And for this to happen, the diesel fuel must be broken up into tiny particles – atomized – and spread throughout the mass of dense, compressed air into which it is being injected. The way this is done is by bringing the fuel to a very high pressure and driving it through a series of tiny holes (orifices) in the tip of an injector (the device that is used to spray the diesel into a cylinder). The higher the pressure and the smaller the orifices the more the fuel can be atomized and the better it can be distributed, resulting in cleaner, more efficient engine operation.

The high pressure fuel pulse that is generated by the injection pump is fed to the bottom of the injector below a flange on the nozzle valve. The pressure lifts the nozzle valve, allowing the fuel to exit the injector via the orifices in the injector base. The top of the nozzle valve presses up on a spindle, forcing the spindle up against a powerful spring. Injection pressures are set by screwing a cap nut down against the spring. During the injection pulse, some of the fuel works its way up past the nozzle valve and spindle to lubricate the injector, exiting via the leak-off fitting.

Cetane Rating

Another factor affecting combustion processes is fuel quality. When the diesel fuel is injected into a cylinder, there is a momentary time delay before it burns. During this time delay, with most injection systems additional fuel is being injected. The longer the time delay the more the fuel that accumulates in the cylinder before combustion takes place, and the greater the explosive force at that moment (at idle, it may be heard as 'knocking'), reducing efficiency and increasing engine stresses.

The combustion speed of diesel fuel is given by its cetane number (CN). The higher the number the faster it combusts and the better it is for clean and efficient engine operation. In Europe diesel quality is defined in EN 590. Since 2001 it has required a minimum CN of 51. Premium diesel fuel can have a CN above 60. North America uses ASTM D975, with a minimum CN of 40 and common values in the 42-45 range. California has a minimum CN of 43. In general, we should be looking for CN numbers in the 48-50 range. In practice, we typically have no control over the cetane number of the fuel we are taking on board (it is whatever the marina supplies) but in general the higher the number the better.

Biodiesel if likely to have a CN of 48-52, although this is  dependent on the source material and production processes.

For optimum performance we want high injection pressures and high cetane ratings. Over the years injection pressures have risen from 2,000 pounds per square inch (psi – 140 bar) to an incredible 40,000 psi (2,850 bar). Pressures are predicted to go as high as 60,000 psi (4,280 bar).

The high-pressure systems use very different technologies to the lower pressure systems. We need to be able to recognize, and understand the core operating principles, of both. First, we will look at the lower pressure systems.

  • This is terrific! I learned so much with the online electronics course, and I am very much looking forward to the complete diesel course!

  • Thanks for the freebie. Really looking forward to the diesel engine course. The electrical i have a decent understanding of but the engine is pure voodoo. Need help soon.

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