Parallel power pays off

Traditionally, larger power requirements have been met with bigger, thirstier and more expensive engines. Increasing numbers of Scania customers, however, are discovering that smaller is better.

TEXT: ÅSA LARSBO

In the power output range above 600-700 kVA, engines become much more expensive as they can no longer be found in the vehicle industry and, accordingly, are manufactured in shorter batches. But, as more and more of Scania’s customers are beginning to discover, the lower cost of the engine is just one of the advantages of substituting one large engine for two or more smaller ones.

“For many applications in all three of our segments – marine, industrial and power generation – the lower purchase price is actually the least significant advantage,” says Pierre Holmberg from Scania’s Application Engineering, a department dedicated to helping customers choose the right installation for their application. “The major gains are in lower emissions and fuel efficiency.”

In power generation, the generator set needs to be able to handle peak output, but this is rarely required in everyday use. “Some generator sets routinely operate at 10-20 per cent of their maximum load, but because engines tend to be optimized to run on 70-80 per cent, large single engine gensets can be inefficient,” explains Mr Holmberg. “And low load operation often results in increased wear and tear on the engine.”

The key is to combine the output of smaller gensets to reach the desired output. Says Mr Holmberg: “Connecting several smaller gensets in parallel with a power management system that ensures that when one engine reaches a load 70-80 per cent, the next one starts, you are constantly staying within the “green window” of the engine. That is the point at which the engine is at its most efficient in terms of load and fuel consumption.”

This principle is also becoming increasingly common in the marine sector, where diesel-electric power is replacing the traditional propulsion system of two large engines, axles and propellers. Diesel- electric power means that a number of diesel engines are connected to generators powering electric engines, which in turn power axles and propellers. This system dispenses with the need for both propulsion engines and auxiliary engines as the power packs take care of both functions.

Power management

A power management system monitors power requirements and starts or stops the diesel engines accordingly. “As in the case of the power generation system, the system is programmed to produce an output of 70-80 per cent from each engine, before the next one starts, and it is often enough to run two or three out of five engines to reach the ship’s cruising speed,” says Mr Holmberg.

Another area where the idea of several small power packs replacing a large one is making inroads is the repowering of diesel-electric locomotives. As many locomotives generally operate on low output most of the time – often only reaching maximum output for 10-15 per cent of their operating time, the main consideration is fuel economy.

“We expect this technology to be very interesting to operators looking to repower their fleets of diesel-electric locomotives,” says Mr Holmberg. “Many operators are still running old two-stroke V16 and V18 diesel engines, which are noisy, dirty and consume large amounts of fuel. The environmental benefits that can be made from repowering these locomotives with several smaller power packs are enormous.”