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How your diesel engine works


Jeremy R. Hood

Being a foreigner I wasn't brought up with the idea that oil could be found in your back yard and so it was exciting for me when I first arrived in Texas and saw herds of those iron donkeys grazing in the fields pumping oil out of the ground. I suppose if you were born here they were so common that you hardly gave them a second glance, their great flywheel and counter weight revolving slowly and the arm continually pivoting in unison, raising and lowering the vertical shaft which was doing the pumping. In some ways they are ugly, scaring the countryside yet their slow but persistent motion perhaps reflects the stubborn tenacity of those Texans who worked long days and nights to make their fortunes here.

These oil pumps which surely everyone must be familiar with, native Texan or otherwise, are a good example of how single cylinder diesel (or for that matter gasoline) engines work. On the oil pump a flywheel and counterweight are driven around, often by an electric motor, and with each cycle the pump in the ground is raised and lowered once. An engine works in an extremely similar way, though in reverse. The part going up and down is the piston and this connects to the flywheel via the crankshaft. On the oil pump the flywheel is driven around and provides power to the pump; in an engine it is the piston being forced down by explosive gases that causes the crank and flywheel to rotate.

When recreational sailing was in its infancy many sailboats had no engine at all and many accounts can be read of sailors drifting around in the currents for several days waiting for favorable wind to allow then to make port. Those boats equipped with an engine must have been the luxury yachts of the day though it is hard to imagine it so. In his first book, The log of the Maken, Ian Nicholson, the Scottish naval architect and designer who has written extensively about small boat sailing, describes how, on his first real passage, the one cylinder diesel engine had to be started. First the kerosene stove had to be lit and from that the blowtorch:

" ...Then, carrying the torch aloft, and without singeing more than the minimum amount of paintwork, the operator would go up on deck and climb down the slippery vertical ladder into our Black Hole of Calcutta, the engine room.

The engine room was not a pleasant place, It lacked both light and ventilation. In the center stood the massive single cylinder, a tower of rusty steel coated with oil, grime and soot. Various pipes meandered about with disconcerting lack of straightness and some sharp bends which were rank bad engineering. At the fore end of the engine was the flywheel. If everything else on the Maken was over-size the flywheel outvied all the other gear. It lurked deep down in the bilge, a thick circle of steel so ponderous in appearance that at first glance one could be excused for wondering if any human hand could turn such a vast piece of ironware. In practice even Mike, for all his strength, couldn't turn it; at best he could rock it gently backwards and forwards. But we are getting ahead of ourselves.

Down the engine room ladder our operator directs the blowlamp on to the top of the engine. The idea is to get the top of the cylinder thoroughly hot. Meanwhile a second victim has climbed into the pit of horrors and is oiling various points on the engine. There are, dotted about in the most remarkably inaccessible places, little brass bowls, like egg cups, which have to be filled with oil from a battered old can. Doing this in the dark, with the ship pitching in a seaway, it is not surprising that plenty of oil was spilled into the bilge. Naturally the risk of an engine seize-up was not to be considered, so we were liberal with the oil. This merely doubled the quantity which went into the bilge.

With the top of the cylinder almost glowing hot, one victim would pump the fuel into the cylinder while the other rocked the gargantuan flywheel backwards and forwards. If all went according to plan and prayer there would be a thumpety-thumpety-thump and the engine would go at once and for ever till the fuel ran out. That was the chief virtue of this primitive old grinder. It was reliable to the point of embarrassment..."

Fortunately for us such seasickness inducing tasks are rarely required though the description does illustrate many essentials about diesel engines. The difficulty in getting them started, the need for pumping fuel and for lubrication. And not least their reliability. It is for this last reason, along with the less dangerous use of diesel over gasoline, that makes us have such an engine in our boat. But just how do diesel engines work? And how different are they from the gasoline engine that is in our car or truck?

The essentials

Just as in a gasoline engine, there is a piston or series of pistons connected to a crankshaft. The pistons slide up and down inside a cylinder which is sealed at the top with the cylinder head. Fuel is introduced into the top of the cylinders and then ignited. The resulting explosion forces the piston back down the cylinder and this turns the crankshaft. When there are several cylinders in-line the explosions are arranged to occur sequentially so that at any given moment the crankshaft is being turned round by the force of a piston being pushed down the cylinder. While one piston is working the others are either compressing (and hence heating ) the air above the cylinder, or discharging the burnt fuel via the exhaust system. Thus there are four stages to the process: induction, when air is introduced above the piston; compression when it is being forced into a much smaller area and as a consequence being heated; power when the fuel ignites and the piston is forced down the cylinder; and exhaust when the burnt gases are expelled. Most diesel engines have a four-stroke operation which means that each of the four stages takes place on its own stroke. As the piston goes down inlet valves in the cylinder head open and induction takes place, as it comes back up the valves close and compression occurs, as it goes down again the power stroke takes place and then, on its way back up for the second time the exhaust valves open and the gases are discharged. In all of this gasoline and diesel engines are similar.

Differences between diesel and gasoline engines

I recall when I was a teenager and went to work on a farm how I was taken advantage of by the older farm hands being sent to get a left handed screwdriver, or pigeons milk or to change the spark plugs on the old farm tractor. The old farm tractor had a diesel engine and hence no spark plugs. In gasoline engines the fuel is introduced by the carburetor on the induction stroke, is compressed and then just at the right moment a spark from the spark plug ignites the fuel and the power stroke occurs. Diesel, like gasoline, will explode by itself if heated sufficiently. In your car, care is taken to ensure that spontaneous combustion does not occur as it will damage the engine if the explosion occurs before the piston reaches the top of the cylinder where it is then deliberately ignited by the spark plug.

In a diesel engine it is not fuel but merely air that is compressed. But the compression is much greater than in a gasoline engine and the compressed air at the top of a piston will be at a pressure of over 400 pounds per square inch (p.s.i.) and because of this high pressure it will have become heated to over 850? F. Then at a precise moment diesel fuel is squirted into the space above the piston where at this high temperature it ignites. This is perhaps the biggest difference between the two types of engine and will explain why some of the other differences occur. On a gasoline engine the fuel is sucked in on the induction stroke and because of the suction a simple measuring device such as the carburetor is all that is required. In a diesel engine the fuel is not introduced until the piston is near the top of the compression stroke and the pressure is extremely high. To force diesel fuel into the cylinder at this time requires an even greater pressure than exists in the cylinder and for this reason injectors and an injector pump are required. Typically the injection pump will force the fuel into the cylinder at a pressure of over 1,500 p.s.i. Because of the high pressures everything on a diesel engine has to be built more heavily and more strongly than on a gasoline engine.

More about fuel

Because diesel engines don't require spark plugs and hence no electrical system to run them they are inherently more reliable especially in a marine environment where salt water and electricity do not do well together. But because they don't require an ignition system they do need a high pressure injection system. Injectors and the injection pump are the heart of your diesel engine and if either fail so will the engine. Because of the high pressures required both the injectors which fit into the cylinder head and the injection pump which delivers a precisely measured quantity of fuel at high pressure to the injector, have to be engineered to extremely high tolerances. Neither are able to be serviced or repaired without special equipment. Any slight mismatch in sizing or any scratches may result in them not being able to build up sufficient pressure and without this, fuel cannot be introduced into the engine. It is for this reason that special emphasis is placed on cleanliness in the fuel system as foreign particles entering the injection pump can very easily damage it. Whereas in a gasoline engine some dirt can be perhaps tolerated (though not desired) in your diesel engine it can, and often will, result in failure of the engine with perhaps expensive bills. It is for good reason that these engines are equipped with several filters to clean any impurities from the fuel.

Firstly it is important that the fuel be relatively clean when you put it in your tank and in areas of the world where less attention is paid to this than here in the U.S. it is sensible to use a funnel with a filter gauze in it to stop the larger pieces of debris entering your tank. From the tank, fuel normally passes to a primary fuel filter which is often combined with a water separator to eliminate any water from the fuel (water won't burn but will cause rust!). This primary filter will probably stop particles of dirt larger than 10-12 microns (1 micron= 1 millionth of a meter).

After the primary fuel filter comes the lift pump which is usually a mechanical pump mounted on the engine which operates at low pressure sucking the fuel up from the tank and passing it on down the system. Because this is a mechanical pump and only works when the engine is running you will sometimes find an electric pump installed between the primary filter and lift pump. This electric pump should normally only be used to pump fuel when the engine is not running such as when changing filters and trying to eliminate air from the system (air won't burn either and can often cause a blockage in the injector pump). After the lift pump comes the secondary filter which is normally mounted directly on the engine. Fuel passing through this filter is further screened for even finer particles of dirt before it is passed on to the injection pump.

The injection pump passes the fuel at high pressure to the injectors in a precise sequence and with a timing to match the position of the pistons in the cylinder. It normally operates efficiently and without failure for many thousands of hours so long as the fuel entering it is kept clean. The fuel itself lubricates the pump as it is being passed through.

When fuel reaches the injector it compresses a precisely set spring within the injector allowing fuel to spray in to the cylinder. The exact design of the cylinder head, piston top and of the hole or holes in the injector determine the efficiency of the engine, the aim being to distribute the fuel as evenly and as quickly as possible. In the injector a small quantity of fuel is allowed to lubricate the mechanism and this fuel is then returned either to the injection pump or to the secondary filter via a line connected to all the injectors.

Because both the injector pump and injectors use the fuel as a lubricant more fuel is delivered to them than is necessary and the unused fuel is eventually returned to the fuel tank along a return line.

Because of the precision required and the complexity of the system most problems that occur with diesel engines are caused by the fuel system and the main ones will be discussed in next months article on servicing and repair.

The exhaust system

The exhaust system of your diesel engine differs from that on your car not because it is diesel but because it is a marine engine. Exhaust gases are blown out of the engine by the rising piston on the exhaust stroke and exit through the exhaust valve on each cylinder. All of the exhaust valves exit into the exhaust manifold to which is joined the exhaust hose. This leads to the exhaust box or muffler and the outlet of this leads, via a loop to take it above the water line, overboard. In order to cool the exhaust gases and to assist in sound reduction, sea water from the cooling system is injected in to the exhaust hose and is then discharged through the muffler and overboard. Because the exhaust is water cooled neither the hose nor the muffler is exposed to high temperatures and on most modern sailboats the muffler will be made of fiberglass or a rubber type compound though stainless steel and other materials are still reasonably common. The muffler operates by allowing a build up of water in the base of the unit which causes a slight back pressure in the system with a resultant sound deadening effect. Once water has built up sufficiently, the exhaust pressure from the engine forces it out of the exhaust and a new build up of water begins. It is for this reason that water exiting the exhaust is seen to come out in spurts rather than as a continuous stream which may otherwise have been expected.

The design of an exhaust system is crucial not only for the efficient running of the engine but to avoid the possibility of seawater finding its way back to the engine which will almost always result in expensive repairs. Water from the cooling system must be injected in to the exhaust hose below the level of the manifold (even when the boat is heeled) and a good installation will take the cooling water above the water line in a loop with an anti-siphon valve at its peak before it is injected into the exhaust. The exhaust from the muffler itself must rise well above the waterline to avoid the possibility of sea water siphoning back into the engine and ideally the through hull will incorporate a sea cock which may be operated in severe conditions to ensure that a following sea is not forced up into the exhaust and thence to the engine.

The cooling system

Like the exhaust system, the cooling system is different from your car only because it is a marine installation. Essentially the two start out the same, with fresh water circulating around the cylinders to take away the excessive heat. On your car the resultant hot water passes to the radiator where it then cools because of the cooler air passing over the fins.

In a marine installation the circulating fresh water is cooled more efficiently than in a car by the use of the surrounding water (often referred to as raw water) on which you sail. However the two types of water never actually come in to contact but heat is transferred from the circulating fresh water to the raw water via a heat exchanger. This is often seen mounted near the top of the engine above the exhaust manifold. Sea water from a through hull below the waterline passes to a raw water pump and into the heat exchanger, exiting at the other end and passing into the exhaust. Within the heat-exchanger is a unit similar to a cars radiator through which the fresh water is pumped. This type of cooling is known as a fresh water cooled system and is the most common type encountered in modern marine diesels. However in some engines such as the older and smaller Volvo engines (such as my boat has) the raw water itself circulates around the cylinders before being expelled through the exhaust. The system has much to recommend itself for simplicity but the disadvantages also are considerable. Having salt water circulating through your engine could cause serious electrolytic corrosion (electrolysis) if different types of metal were encountered and so a raw water cooled engine must be made of all one type of material; usually cast iron. Another consideration is that compounds such as salt, minerals or pollutants which are dissolved in the surrounding water may become caked around the engine as happens in a kettle. Within a short time the water-ways through which the cooling water passes could become clogged and the engine overheat. To overcome this to a large extent, the operating temperature of a raw water cooled engine is kept below about 140?F which minimizes the problem though it then means that the engine is not operating at its most efficient temperature. Yet another problem with this type of installation is the difficulty of ensuring a stable operating temperature as this will depend to a much greater extent on the temperature of the surrounding water.

In a fresh water cooled engine, the raw water only passes through the heat exchanger where, if sufficient flow occurs, it will stay relatively cool. With a good raw water strainer at the sea water intake to filter out large debris the heat exchanger will be useable for many years and will often then only require a good cleaning. The fresh water which circulates around the engine can only ever deposit its one load of minerals and so this problem is largely eliminated. In addition anti-corrosion additives and anti-freeze may be added as in your car. The temperature of this water may be controlled easily with a familiar car-type thermostat which reduces the water circulation until operating temperature is reached, then opens, to allow full circulation and greater cooling. Being able to run the engine at higher temperatures gives increased fuel efficiency and a longer life to your engine.

Electrical systems

In his description of starting the old diesel engine aboard the Maken, Ian Nicholson makes no mention of electrical equipment for the simple reason that a diesel engine requires none. However we live in an age where we do not want to be bothered with hand starting an engine and we need electricity aboard our boats for lights, electronics and other essential equipment! For these reasons a diesel engine will be fitted with an electrical system though by any standards it is rudimentary.

First there is a starting system. Aboard the Maken a blowtorch was used to preheat the engine prior to starting. On many modern engines an electrical heater is used to preheat each cylinder. These heaters, called glow plugs, fit in to the cylinder head and are activated by a switch usually on the engine panel. By preheating each cylinder the compressed air will more easily reach the temperature required to ignite the diesel. Usually these glow plugs are activated for about 30 seconds before an attempt is made to start a cold engine. Rather than turn the flywheel by hand, an electric motor is arranged to engage with the flywheel and to turn the engine over until the compression in the cylinders is sufficient to cause the diesel to ignite and the engine to start. The starter motor will normally be powered via a starter solenoid mounted on or near the motor. The purpose of the solenoid is to allow the thick, heavy wires necessary to run the motor to be led almost directly to it. The solenoid is merely an automatic switch which connects these thick wires directly to the motor when the ignition button is pressed.

Separate from the starting system (except in some very old and small engines) is the charging system. An alternator will be driven by a belt from the engine and will produce electricity while the engine is running. This electricity will be used to charge the ships batteries and to run your ships electrical equipment. Normally the alternator output will be led via a battery isolator to each of your batteries. The isolator allows each battery to be charged independently without the risk of one dead battery draining the others: each is kept isolated yet charged.

The remainder of electrical wires going to a diesel engine are mostly to monitor what is happening. There will be one wire going to a temperature sensor to monitor the fresh water cooling system. The sensor wire may go to a temperature gauge or just to a high temperature warning light. Another sensor will monitor oil pressure and be displayed either by a gauge or, a low pressure warning light.

While all modern marine engines will have a starter system, charging system and temperature and oil sensors, and many will have glow plugs, additional equipment may be fitted on some engines. Such equipment may include another solenoid used to stop the engine, or additional wires to monitor the charging current from the alternator. Quite often a wire from the alternator will also be used to run a tachometer to monitor the engine r.p.m.

Understanding how your marine diesel engine works is essential if you have to undertake repairs at sea. There really is nothing complicated about it and, as will be discussed in the next article, there are many simple tests that you can make to diagnose problems with your engine. But the real key to diesel engines is regular servicing: with a little preventative maintenance your marine diesel, like the one aboard the Maken can be "reliable to the point of embarrassment"!

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