Here is a guide to the amount of diesel that small marine engines use. The grammes
per hour figures are precise; other figures are strictly a rough guide. European-based: A general base figure for the fuel consumption of a modern marine diesel engine is: 180 gm / hp / hour. This is 1.8 kg / 10 hp / hr, or 2.1 litres / 10 hp / hr, or 0.21 litres / hp / hr. Note that older engines will not achieve these figures. UK-based: .034 gal / hp / hr, or .34 gal / 10 hp / hr. U.S.-based: .4 lb / hp / hr, or 1 US gal / 18 hp / hr. Diesel weighs 1 kilo per 1.18 litres; 1 litre = .85 kilo; or, about 8 lbs per gallon (freshwater is 10 lbs per gallon). Don't you just love the way manufacturers quote fuel consumption by weight – who cares what it weighs? Well, that's not strictly true of course: commercial fishing boats buy it by the tonne. But there's more of us (now) than there are of them... _____________________ Small diesels Generally, small diesels are less economical than these figures, typically at around: 200 gm / hp / hr or more. They may seem better – but that's because they are using a lot less in any case. Older engines will also use more diesel. Large diesels Often better consumption than the average, at around 170 / gm / hr. Turbodiesels Also better, often around 160 gm / hp / hr or better. Large turbodiesels A large turbodiesel of modern design can be even better at around 150 gm / hp / hr. A large oil-cooled turbodiesel can make 140 gm / hp /hr. _____________________ Rough guide Anyway, very few of us are about to go out and buy a large new oil-cooled turbodiesel – it won't fit in the Seawych. So what's your engine probably going to use? At low cruising speed, consumption is roughly .25 gal / 10 hp / hr. This means you are probably using about 5 to 7.5 hp out of the 10 available; it is not the same as saying consumption is X per hour for a Y hp engine – at either low or normal cruising revs, you are not using all the hp of your engine (if you had to, your engine and/or prop isn't big enough). Also, any preferred cruising speed is not top speed, hull speed, or full revs – at any of these, whether or not they are the same, consumption drastically increases. Cruising speed is often taken to be achieved at about 75% of full revs, though this depends on a lot of other factors. _____________________ Cooling The method of cooling the engine also has an effect on fuel consumption. Since there is no ignition system on a diesel engine, the fuel is ignited by heat and pressure. The colder the engine therefore, the less efficient. The hotter the engine runs, the less fuel it will use. Of course, there is a cap on the heat level: cooling water cannot be allowed to reach its boiling point; and oil-cooled engines must stay below the point at which heat will damage the oil; the first casualty then would be a turbocharger, if fitted (they run at around 20,000 rpm so a good supply of clean oil is critical), followed by gaskets, then the rest of the engine. Raw-water cooled engines, where seawater runs through the block, run at 60° C, and are kept down to a lower temperature because at hotter temperatures the saltwater is highly corrosive. Freshwater cooled engines (where the engine block and a header tank contain freshwater, cooled by seawater via a heat exchanger) run at 90° C, the maximum temperature viable by this method. Note that in both of these watercooled cases, the running temperature is not the temperature at which the thermostat opens, which for a freshwater engine is typically 82° C. Oil-cooled engines, where the engine oil is cooled by seawater via a heat exchanger (this water is then injected into the exhaust as per usual) run a little hotter still. In northern Europe, they are undoubtedly a very good choice. This type of engine is in reality oil and air cooled, and they always benefit from additional cooling air throughput (like all engines – but more so). Therefore in terms of fuel use per hp, a raw-water cooled engine is least efficient, a freshwater engine better, and an oil/air-cooled engine best. Turbocharged engines are usually very efficient. They are always freshwater cooled. An intercooler can be added, to increase its efficiency further. Final result poor: small raw-water cooled engines good: large freshwater cooled engines better: large turbo-charged engines excellent: large turbo-charged oil-cooled engines _____________________ Air consumption An engine needs a certain volume of air for combustion, and a certain amount for cooling. Combustion air is sucked into and through the engine, and exits via the exhaust. It needs to be freely available in the engine space, via air vents to the transom, deck, cockpit, or cabin. Cooling air also needs to be available in the engine space. However, it will also need to exit via vents, so that it forms a continual air current passing through. Opinion varies on the amount of air throughput for cooling, as opposed to that for combustion. Amounts quoted vary from 50% of the combustion air, to 100% or an equal amount. This should exit via fans or vents; though according to some sources a proportion of cooling air throughput can go out as combustion air. It seems that some external venting should be provided for cooling air exit; but it also seems unwise to fit a fan for this purpose unless air input capacity to the engine space is more than sufficient, since otherwise engine air starvation will occur, and the fan will be stalled, causing eventual burnout. In addition, a fan input for cooling or combustion air seems best avoided unless there are other and alternative input vents (in which case a fan does not seem necessary), since engine suction may cause the fan to overspeed. These considerations seem to indicate the provision of large vents for both input air and hot air exit. Due to the high volume of air that is required by an engine (and more as engine size increases), many engines probably suffer from air starvation to a greater or lesser degree. The alternator is often a casualty since it is more vulnerable to overheating than most other parts, which is why it's a good idea, if air is ducted in rather than simply being available from vents, that the air ducting plays onto the alternator. A figure quoted for air requirement is 45 m³ / 10 hp / hr; but it hardly seems possible that this volume could be provided in many installations. _____________________ +++ TOP TIP +++ If you've just bought a boat with a newly-installed or rebuilt engine this may catch you out, as it did with one of my customers recently: diesels, unlike petrol engines, have two fuel lines to the tank and not just one. This is because a certain amount of fuel is returned to the tank unused. If you find diesel in your bilge, and only one fuel line to the engine, this is the problem you need to solve. In the short term, for any engine of under 40 hp, you can simply connect a flexible pipe any way you like (it's not critical) from the leak-off pipe (aka spill pipe), running down to an empty 2-litre pop bottle in the bilge. The leak-off pipe is the daisy-chained pipe, frequently plastic, at the end of the injector banjo connectors. Air leaks are unimportant here. You can drill a hole in the top of the bottle cap, and tape the pipe to the cap. Some small engines seem to use no extra diesel whatsoever (whatever the theory says); others generate a fair amount. Larger engines should use a gallon can; but even so, there isn't usually a vast amount that will need regularly tipping back into the tank; about every four hours will be necessary, I find. Many thanks to contributors and critics: Alan Gluyas, Aus. - www.petrospection.com.au ^ TOP ^ |
Diesel Consumption Rates |