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GAS ENGINE TECHNOLOGY
Page 63
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THE DIESEL The 12-litre Tx is not the most advanced diesel on the market, but Perkins has developed the old straight-six Eagle to meet Euro2 requirements without electronics, while retaining decent fuel consumption. In CM'S Euro-2 arctic test last year, a Tx-powered ERF returned excellent fuel economy. The Tx is conservative in overall design, with two valves per cylinder and mechanical fuel injection. However,, details such as a turbocharger wastegate (at higher outputs), composite sump and cam covers and a "fly-by-wire" electronic accelerator make it more interesting.
THE GAS ENGINE The Perkins engine in the gas-powered ERF is definitely not a straightforward conversion of an existing design. While it uses the basic structure and geometry of the diesel Tx, the gas engine is dramatically different. The obvious change is that the gas engine is a spark-ignition (SI) unit, rather than a compression-ignition (Cl) diesel—it runs on the Otto cycle (like a four-stroke petrol engine), so it requires sparking-plugs, ignition generation and timing and a throttling fuel/air system. The engine could not really be based on a petrol design, either. For a start, few petrol engines are large enough, but also the high octane rating of natural gas (around 120) encourages a high compression ratio. Gas engines have been run at compression ratios as high as 18:1, much like a diesel.
There is no reason why-a spark-ignition gas engine shouldn't produce more torque than a diesel equivalent—at its best, the gas engine should be able to produce around 15% more torque than the diesel. And efficiency need not suffer: in fact, at wide-open throttle the gas engine should be slightly better than the diesel. The latter is inherently more efficient at part-throttle than the SI engine (because it runs with a lean fuel/air mixture), but fullymapped electronic fuelling and ignition systems can close the gap. The engine fitted to BOC's truck has no such sophisticated controls: it is a prototype on-road design using technology from Perkins' 2006 SI, a turbocharged gas engine—itself based on the Tx—designed for power generation and CHP (combined heat and power) operations. More than 200 of these have entered service since 1992, from Australia to Alaska, running on fuels as diverse as North Sea gas, wood gas and landfill gases. The ERF's engine uses the core of a Euro-1 Tx engine with cylinder heads derived from the 2006, and like the 2006 it runs at a compression ratio of 12:1. The fuelling system uses a "mixer", effectively a gas carburettor. Throttle control is "drive-bywire", and the ignition system is electronic, the low-flame speed of gas requires more ignition advance, which changes throughout the rev range.
Each of the six spark-plugs has its own ignition coil. Future developments will almost certainly use combined coil/sparkplugs for greater simplicity and reliability. The gas/air mixture is highly flammable. This may seem obvious, but there is a rkk of detonation in the inlet manifold when starting up, so the engine is cranked for a couple of seconds before firing.
The engine has an Engelhard two-Nay oxidation catalyst fitted. A three-way catalyst (to cut NOx as well as carbon monoxides and hydrocarbons) would require a lambda sensor to measure the air:fuel ratio, connected to an electronic control sysem.
FUTURE DEVELOPMENTS Perkins admits that the gas engine we tested is a first-generation prototype. There are two clear stages of development to go. First, the fuelling and ignition system will be equipped with fully-mapped, closedloop electronic control, still based on the mixer mechanism. This will allow the engine to run a leaner mixture at part-throttle, increasing efficiency, reducing exhaust 'emperatures and permitting a more effective catalyst to be fitted. Mechanical refinements will include the lighter flywheel to speed gearchanges, a more efficient exhaust brake system and features from the Euro-2 diesel such us a twin-cylinder compressor.
The second stage will be to produce an engine with multi-point fuel injection, like the Phaser 220TiSi fitted to BOC's ERF E C6. This system has the potential to allow an increased compression ratio, improring efficiency under most operating conditions.