ISME Keynote Address 24 October 2000
Heavy Fuel Engines Maintain Their Lead by continued development
Stephen G. DEXTER *
Large Engines and Compressors
Lower fuel costs encouraged most shipowners to burn heavy fuel in their medium speed engines some forty years ago. Many problems were encountered but steady development has been done to overcome these as described in the paper. Heavy fuel engines now have a strong lead over other prime movers as a result.
This lead will be maintained for many years despite new threats from exhaust emissions regulations. This is because emissions can be controlled and there would be a very significant cost penalty if the use of heavy fuel oil by marine engines were to be abandoned.
Key Words: Heavy fuel, marine, engine, cost
Imagine being able to reduce the fuel consumption of a marine engine by 40%. Today it is inconceivable that any concept such as turbo compound, ceramic materials, electronic control or other device could give anywhere near a fuel consumption reduction approaching this. But to those working in the design and development of marine engines in the early 1960s a 40% reduction in fuel cost was the incentive we saw in converting medium speed engines to run on heavy fuel oil (HFO). Certainly there were problems to be faced but with such a large incentive any manufacturer not prepared to overcome them soon found they could not stay in the business. Figure 1 shows the price of HFO.
It has consistently been cheaper than marine diesel oil (MDO), the distillate fuel largely used at sea, so this incentive remains unchanged today. Low speed engines are well suited to burning HFO but medium speed engines present a greater challenge. This is because of the reduced time available for combustion and the fact that the products of combustion come into direct contact with the crankcase lubricant.
In the 1960s the medium speed engine was taking an increasing proportion of the marine propulsion market because of its small size and its lower cost per kW. Ship owners were prepared to remove cylinder heads or valve cages at 5,000 hour intervals to overhaul exhaust valves, and a piston ring life of 10,000 hours was considered to be good. There was no concern about NOx, SOx or particulates and, provided the engine operated reliably between overhauls and the visible exhaust smoke was not bad enough to attract criticism by port authorities, heavy fuel seemed to be the ideal solution for the marine propulsion engine. When the first oil crisis of the 1970s raised the price of fuel there was no going back and even marine auxiliary engines were converted to run on heavy fuel.
A four engine ferry with a total power of 15 MW could use 50 tons of fuel a day and this represents a cost saving of some 5,000 USD per day at today's prices if the engine were to run on heavy fuel. With this incentive the only vessels which could afford to run on MDO were small single engine coastal vessels, where the operating hours were fewer and the cost of the fuel handling and treatment was not economic, or naval vessels where fuel cost is of secondary importance.
But what of the future? We now have NOx, sulphur and particulate emissions to consider so will this situation change? Only a clairvoyant or a politician would be brave enough to give a sure answer to this but we can consider some of the engineering issues to help us see how heavy fuel engines have maintained their lead and which way development might go.
2. THE DEMAND FOR BETTER DURABILITY
Soon after ship owners started to buy medium speed engines operating on heavy fuel in the euphoria of lower fuel costs they discovered that their maintenance costs increased because engine durability was suffering badly. In the last 40 years much has been done to overcome these problems and it is interesting to note the effect it has had on engine design.
2.1 Piston ring and liner wear
The life of rings and liners in heavy fuel engines was less than half of those engines burning MDO. Sometimes hard particles were to blame. These originate from ash in the HFO, products of combustion or, more recently, catalytic fines. Particles in the fuel and in the lubricant have been reduced by filtration and by centrifuging. Harmful combustion products have been tackled by better control of the fuel injection and combustion and by improved turbochargers which provide more air. Carbon formation in the ring pack has been reduced by improved lubricant additive packages .
* AVL List GmbH, Hans-List-Platz 1, A-8020 Graz, Austria
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