Diesel Engines

As many of you will be aware, and will be informed by the majority of the Yacht Brokers, engine inspections are rarely carried out by Surveyors. Generally the majority of Surveyors overlook the engine completely and often don’t include it at all in their reports.

I, not as a technical engineer, but as competent Surveyor visually inspect engines and generator installations during my  inspections, and where possible in commission the engine(s) is run up to access its general running characteristics, vibration levels, etc. No dismantling of the engine or associated equipment is carried out within the scope of a condition survey so no detailed comment upon the internal parts is possible. As I cannot comment on the internal condition of the engine(s). It is always strongly recommended that the engines(s) are examined and further tested by a competent marine engineer familiar with the type of machinery onboard.

Its important to remember, diesel engines can deliver years of trouble-free service- given proper preventative maintenance procedures. Some general information is listed below with some troubleshooting tips and advice to think about while on your sea / river trial.

The majority of diesel engines are remarkably simple in principle. A piston compresses air in a cylinder, and the more the air is compressed the hotter it becomes. When the piston is near the top of its stoke, diesel is sprayed (Injected) into the cylinder of compressed, superheated air and ignites immediately, raising temperatures and pressures even higher, which drives the piston forcefully back down the cylinder.

Diesels are sometimes called compression ignition engines. They do not have a true ignition system. The diesel fuel is ignited solely by the high temperature attained by compressing air. A diesel engine will never run if the compressed air does not reach ignition temperatures.

Given ignition temperature, all it takes to make a diesel engine run is a supply of clean air and correctly timed and atomised injection of fuel dispersed into a cloud of tiny particles. A diesel is hungry for air. Even a small diesel will consume every hour enough air to fill a 20 by 20 foot room! An inadequate air supply will produce in- complete combustion, causing the engine to loose power, overheat, and emit black smoke.

Air filters are sorely neglected. A plugged air filter restricts airflow through the engine, reducing performance. A ruptured air filter will admit tiny particles of dirt. These become embedded in all the soft metal surfaces of the engine, noticeably pistons and bearings, accelerating engine wear. Once embedded, no amount of oil changing and flushing will get them out. However the marine environment is moderately free of airborne pollutants, but never the less, check the air filter from time to time.

A diesel fuel system is a miracle of modern technology. It must meter quantities of fuel as small as a few millionths of a gallon, raise this to a pressure as high as 5,000 psi, and inject it into the engine at a moment in time precise to 0.000015 second-incredibly precise engineering!

Given the close tolerances in the machining of all fuel injection pumps and injectors, a minuscule piece of dirt can do extensive damage. If right from the start the owner gets rid of dirt and water in the fuel, then 90 percent of potential engine problems can be avoided.

Most diesels will run trouble free for thousands of hours given clean fuel, oil, and air. There also maybe one or two grease points needing regular attention and bolts, belts to auxiliary that need to be kept tight (no more than ½” or 13mm of deflection under moderate finger pressure in the centre of the longest belt run). Any zinc anodes in the cooling system will need replacing well before they are consumed. If this turns out to be more than once a season, an electrolysis problem needs solving before a heat exchanger corrodes through or some other expensive damage is done.

Modern diesels run faster, hotter, and at higher pressures than their forebears. They generally have smaller oil capacities, a smaller amount of oil is working much harder. Diesel engine oils are specially formulated to deal with the tougher operating conditions, but even so must be changed far more regularly than petrol engine oils. Failure to carry out oil changes will lead to a build up of carbon sludge, dirt and acids in the crankcase; thus build up is difficult to remove even after re-peated flushing.

The air system comprises both the inlet and exhaust side of an engine. Any obstruction in either side will interfere with the air and gas flow through the engine, thereby reducing performance. Some engines do not have an air filter, but most do. Where fitted, the filter needs checking periodically. Just as with a car, it is not possible to lay down hard and fast rules on when to change a filter element-in dusty environments, filters need replacing regularly, but in most marine environments, changes will be rare.

The fuel, even if clean when taken onboard, can become contaminated in the fuel tank. Moisture from the marine environment condenses and mixes with the fuel. Bacteria can grow in diesel, so its advisable to take regular samples from the bottom of your tank, either via a drain or by inserting a pump through the top of the tank. If water is present, the tank must be drained or pumped. If bacteria are present, the fuel will smell and there will be slimy deposits on the tank sides and in the filters.

Any respectable diesel fuel system will have two filters-a primary filter mounted between the fuel tank and the engine, and a secondary filter mounted on the engine between the fuel lift pump and the fuel injector pump.

The primary filter is designed to remove water and larger particles from the fuel; the secondary filter should catch any remaining microscopic dirt particles and suspended water droplets. If your engine does not have a primary filter, get one fitted.

Diesel engine exhausts should be clear with the possible exceptions of:

Generally, any other smoke is a sure sign of problems. The colour of the smoke is a useful guide to the source of the trouble.

Black smoke results from inadequate combustion of the injected diesel. This can arise from a restricted air flow through the engine. (plugged air filter, defective turbocharger, or blocked exhaust);too much fuel injected (generally due to overloading-the governor responds by opening up the fuel rack and pumping in more fuel). Or improper fuel injection (an injector fails to atomise the fuel correctly, dribbles fuel into a cylinder after the main injection pulse, or injects to late).

Check the air filter first. If the engine has a turbocharger, check all the ducting for air tightness. Remove an inspection cover and check the compressor assembly for carbon build up. If you find build up, clean the assembly, making sure it spins freely with no binding. Open up the exhaust line for any kinks or other restrictions.

If the air flow is deemed adequate, what about overloading? Is a line wrapped around the propeller? Are you powering hard into a head wind. Has any extra load been placed on the engine recently, such as belt driven auxiliaries equipment, a high output alternator, or a new propeller?

In the case of faulty fuel injection, remove the defective injectors and send them in for servicing. Make no attempt to work on the injectors yourself.

Blue smoke comes from burning oil. There are only a few paths by which oil can find its way into the combustion chambers-up past the piston rings; down valve stems; through defective turbocharger seals; and out of crankcase ventilators, where there is high crank case pressure as a result of defective piston rings.

White smoke is indicative of one or more cylinders misfiring, water or air in the fuel, or water in the cylinders (most likely from a blown head gasket or cracked cylinder head). If the smoke occurs on start up and at light loads but clears when the engine warms, it maybe due to condensation or water vapour formed in combustion and is acceptable, but then again one or more cylinders also may have a compression problem and be failing to reach ignition temperatures until the engine warms up. If the smoke develops during normal operating, generally accompanied by erratic misfiring, the engine is running out of diesel or has water in the fuel.

Most diesels run unevenly at idle speeds, even though not “missing” with a fair amount of knocking and clattering. This is normal. Under load however, the engine should run smoothly. Rhythmical misfiring indicates one or more cylinders missing all the time; erratic and random misfiring, a generalized engine problem. If the missing is more pronounced at high loads, the fuel filters are probably plugged; if it is accompanied with black smoke, the air filter is also likely to be plugged.

In the case of rhythmical misfiring, the offending cylinder(s) can be isolated by loosening the injector nuts in turn (with the engine running) until fuel spurts out. If the engine slows or changes its note, this cylinder is ok. If no change occurs (or no fuel comes out) this cylinder is missing. NOTE: Don’t do this with a Detroit Diesel; fuel will flood out.

Engines are air cooled (rare in marine use);raw-water cooled (seawater is circulated directly through the engine-also rare);or heat-exchanger cooled. Engines with heat exchangers have an enclosed “freshwater” cooling circuit with a header tank. The cooling water passes through a heat exchanger, which has sea water on its other side, carrying off the engine heat.

Heat exchangers are either inside the boat, complete with their own raw water pump, or fitted to the outside of the boat in direct contact with the seawater (called a keel cooler and quite common on the old Broads cruisers) and so requiring no raw water pump.

Almost all engines with a raw water pump and circuit pass the raw water through any oil coolers (in the engine and hydraulic gearbox, if fitted) and then discharge it into the exhaust to cool and silence the exhaust. Even some keel cooled engines have an extra raw water circuit specifically to cool the engines oil and exhaust.

Check the oil level. If a low oil level is causing the engine to over heat, expensive damage may be in the making.

The raw water inlet screen (if fitted) on the outside of the hull may be blocked with a piece of plastic. Throttle down, put the boat in reverse, and throttle up. With any luck the reverse propeller thrust will wash it away. Check the raw water filter. If there is a lot of debris/silt, the heat exchanger (or engine itself on raw water cooled engines) may well be silted up. Many heat exchangers have removable end caps and can be rodded out with a suitable wooden dowel. Perhaps the engine is overloaded.

The thermostat may be malfunctioning (some raw water cooled engines don’t have thermostats). It will be found under a bell housing near the top and front of the engine. Take it out and try operating without it. To test put it in some water and heat it. It should open at around 165F to 180F except on raw water cooled engines, in which case it should be set to open at between 140F and 160F.

All raw water circuits should incorporates Zinc pencils to keep corrosion at bay. These must be replaced at regular intervals. Failure to do so will result in electrolysis in the heat exchangers and oil coolers, and excess scale formation in the raw water circuits.

Where the raw water is injected into the exhaust (in both raw water and heat exchanger cooled engines) a relatively small nozzle is sometimes used to direct water down the exhaust pipe and away from the exhaust manifold. When scale forms in the raw water circuit, these nozzl3s tend to plug.

If a boat has been operating in salt water and then moves into freshwater, scale formed in salt water will swell and the engine will gradually overheat. On heat exchanger cooled engines with galvanized exhaust piping, check the pipes for a partial blockage. On raw water cooled engines, first look at the raw water side of the exhaust manifold-a major descaling may be in order!

Conclusion-Engine Surveys

Engine surveying requires a great degree of knowledge and skill, along with near constant continuing education. Such knowledge and skill does not come cheaply.

Due to the huge rise in Indemnity Insurance this year, and the fact that a number of good engineers have stopped doing engine inspections for this very reason, there do not seem to be many marine engineers left on the Norfolk Broads with the requisite training and experience. My final advice is to hire the very best engine man you can, and be prepared to pay accordingly.

For an independent engine trial and engine report email: bms@broomboats.com

or phone: Gary at Marine Engineering, Boat Repairs and Service
Brundall Marine Services Ltd
Tel:01603 714372 Fax:01603 716013