The availability of small, rugged, industrial air-cooled petrol engines has revolutionised the design of portable fire pumps over the last decade.
Lightweight precision alloy castings keep the weight to a minimum and the traditional ‘V’ twin layout reduces the length of the engine providing a compact power package.
Forced ventilation air cooling means it is no longer necessary to have heat exchangers to cool the engine water, with the attendant problem of overheating if it runs dry during operations. Add on simple servicing, with most key parts easy to access on the outside of the engine, and it is easy to see why these power units are so popular.
Some technical aspects
Traditional pump designs use a separate shaft for the pump impeller, usually supported on two bearings, with a flexible drive coupling between the pump and the engine crank shaft.
In addition, some designs, especially flat twin engines where the exhaust pipes are at opposite ends of the engine, need power to drive a mechanical priming pump.
In order to do this it is usual to fit a power take off and clutch between the engine and pump body.  The result is a long, heavy, mechanically complex design with multiple bearings, a clutch and a flexible coupling.
Pumps using modern compact ‘V’ twin engines offer the clever designer the opportunity to incorporate additional features not always obvious to the user. Because the engines are designed for industrial use they can be supplied, as standard, with an extra long crankshaft. In addition, the engine crankshaft main bearing at the power output end is normally a heavy duty roller bearing large enough to take side loads.
This, combined with careful design of the pump casing and shaft seal, means it is possible to mount the complete pump and impeller on to the engine crankshaft. Immediately the designer has avoided the necessity to build in a separate shaft, flexible coupling and separate bearings for the pump. The complete pump assembly can rely on the engine crankshaft for support.
The final piece of the design relies on using exhaust gases from the ‘V’ twin to provide priming and avoid the need to power a mechanical primer. Because the exhaust ports of the engine are close to each other they are easily combined to provide a strong flow of exhaust gases to power an exhaust gas ejection priming pump.
Compared to a mechanical priming pump the exhaust primer is simple, has no moving parts when working, and is lightweight.
Removing major sets of components from the design saves weight, complexity, cost and space. At a stroke the pump is more robust, lighter and easier to service.
Other advantages
The industrial ‘V’ twin engine provides other advantages, many of which are critical to the specification of a good fire pump. The engine package is fitted with an electric starter, but because it has a low compression ratio, it is easy to hand start using the emergency pull rope built into the engine.
Each engine is provided with electronic ignition powered by a flywheel mounted magneto and a separate generator for charging the battery. This means the pump can be started even if the battery is flat or missing completely.
Modern fire pumps need to comply with emission regulations. All “V” twin air cooled petrol engines used by Angus Fire comply with current EEC regulations on exhaust emissions and also comply with the stringent California Air Research Board (CARB) regulations - considered to be the toughest in the world.
Other innovations from Angus Fire mean its pumps are continually improving. New Gel Cell batteries introduced in 2006 use a gel as an electrolyte and not a liquid. This means the battery is less susceptible to vibration and can be operated at any angle - even upside down.
Angus Fire has taken full advantage of the ‘V’ twin design philosophy to produce the LW800 portable fire pump.  Since its introduction four years ago the LW800, and its sister pumps - the LW500 and LW1200 - are now the mainstay of the UK’s fire service pump fleets.

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