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The basic principle of a diesel engine is less complex than that of a petrol engine. No spark plug or ignition system is needed, making the basic diesel engine a comparatively straightforward system that results in fewer faults and has lower maintenance costs than a petrol engine.

A diesel engine works as follows. A piston traps a quantity of air in an enclosed cylinder, then rises up the cylinder and compresses the air. As the air is compressed into a small space it heats up to a very high temperature, of about 600ºC. Diesel fuel is injected under high pressure into the small space. The fuel ignites when it comes into contact with the very hot compressed air. Bang! The resulting expansion of gases pushes the piston back down to the bottom of the cylinder, creating a powerful force which is used to drive a crankshaft. As this happens, the piston is pushed back up the cylinder and the exhaust gases escape through an exhaust valve and then the process or cycle begins again. This is known as the four-stroke cycle – literally four strokes of the piston – consisting of induction, compression, power and exhaust.

Valves

In order to get the air into the cylinder and the exhaust gases out of it, a combination of valves open and close through the cycle at precisely the right moment to either allow air in or exhaust out. When they are closed the valves need to be as tightly shut as possible to prevent the air, mixture and gases from escaping at the wrong moment. Strong springs called valve springs are used to hold the valves closed; the valves are opened by other lever type components called rockers. All the opening and closing of the valves is controlled by a camshaft which is connected to the crankshaft. The correct timing of the valves opening and closing is crucial, as is the gap or clearance of the valves.

Things get a little more complicated depending on the design of specific engines, number of cylinders and variations of ways the camshaft is driven – some use chains, some use gears and some use belts. It is probably best to hang onto the principle here rather than get too bogged down in the detail.

Transmission systems

Most inboard engines of sailing vessels use one of two ways to connect the engine to a propeller, either a conventional propeller shaft with a stern gland or a saildrive transmission system. Many mid-range power boat designs have sterndrive systems:

  • Conventional propeller shaft system – a coupling attached to the rear of the engine gearbox connects horizontally to a propeller shaft. The shaft goes in a straight line at a shallow downwards angle to the stern through a stern gland and exits the hull to the propeller. The stern gland prevents the ingress of water while allowing the propeller shaft to turn the propeller.
  • Saildrive transmission system – a saildrive has a transmission system leading from an inboard engine which has a horizontal output shaft. This connects with the saildrive transmission which has a system of gears that drives an intermediate shaft vertically downwards through the hull to more gears that drive a horizontally mounted propeller. This system is sometimes referred to as an S-drive or Z-drive transmission due to the way the shaft is configured.
  • Sterndrive system – sterndrives are also known as outdrives and inboard/outboards. As the names imply, these systems have an inboard engine which connects through the transom to a drive system that drops down via a shaft and gearbox to connect with the propeller. Unlike a saildrive, sterndrives can steer a boat by pivoting, similar to how an outboard engine works, with no separate rudder required for steering.

Evolving technology and regulations

Boat engine technology and regulations have evolved considerably in recent years. It is a complex subject, made even more complex by the different marine exhaust emissions standards in force around the world, resulting in widely different rules that now apply for both commercial and leisure vessels in different territories.

As an example, at the time of writing a mechanically controlled diesel engine may comply with the EU Recreational Craft Directive (RCD), but would not meet the US EPA domestic marine standard as it does not have electronic control and a common rail fuel system. While the DIY maintenance of mechanically controlled diesel engines is within the grasp of many boat owners, common rail technology is much more complex and requires professionals to carry out the maintenance and troubleshooting of these engines.

Diesel engine technology

Common rail technology is a digitally controlled very high pressure fuel injection and sensor system. This results in greater engine efficiency and reduced emissions than with mechanical systems, which is of course of environmental benefit and therefore a good thing. However, in the event of a breakdown out at sea, this is not such good news unless you happen to have a professional engineer aboard with electronic diagnostic testing equipment to troubleshoot any problems that might arise.

These advanced engines are not DIY friendly unless you are a qualified engineer, but this is the way the world is going and if marine diesel engine technology is to survive well into in the future then it will have to meet the increasingly challenging emissions regulations needed to protect the environment. It will also need to compete with the rapid advancement of alternative technologies, in particular electric drive systems that do not produce exhaust gases or carbon emissions during use.

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