Mercury’s diesel progress

US outboard motor manufacturer Mercury has developed a diesel outboard that is rated at 175 hp making it one of the most powerful diesel outboards now available.

The outboard has been developed by the Mercury Racing Division of the company and this launch makes Mercury the first of the major outboard manufacturers to develop a large diesel outboard. Prior to this news, diesel outboards have been developed by small companies that do not have the marketing and support that the major companies can offer.

This new diesel outboard is not a true diesel because the engine has spark ignition but it can run on diesel fuel. The diesel version is based on Mercury's 3 litre Optimax petrol outboard and retains many of the features of the base outboard including variable rotation, V-6 layout and two stroke operation. Mercury say that the main modifications required for diesel fuel operation are in the fuel injection system and in the ignition.

The prototype unit of this outboard was developed by E.P.Barrus Ltd. in the UK and it was exhibited at the Seawork exhibition in Southampton last year. The concept has now been further developed and extensively tested by Mercury Racing before being offered onto the market. At present the unit does not meet RCD and US emission requirements and so it cannot be offered into the leisure and commercial markets. However this may change because Mercury is currently looking at ways to reduce the emissions levels in order to see if they can be reduced to meet the various prevailing standards.

At present the prime demand for the development of this diesel outboard has come from the US military where it has been mandated that no gasoline shall be carried on board US military ships. It is expected that other military operators around the world will also be in the market for this engine.

The new outboard uses a two-stage direct injection system with a small charge of compressed air which finely atomises the diesel fuel as it is injected into the combustion chamber. Atomising the fuel in this way allows for it to ignite using the spark from the plug and a glow plug is fitted to improve the cold weather staring and to improve the idling. A Propulsion Control Module micro-processor optimises the fuel and spark timing for improved running quality and performance throughout the engine speed range. The engine is fitted with Mercury's Smart Craft system for monitoring and diagnostics.

E.P.Barrus has also developed a prototype 44 hp diesel outboard based on a two cylinder Mercury unit that produces 50 hp in the petrol version. It seems likely that Mercury will also be offering a spark ignition diesel outboard of this lower power into the commercial market in the near future.

Visit EP Barrus (Mercury’s UK distributor) at Seawork International 2015 on stand Q2.