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Natural Gas Fuelled Diesels

Running diesels on natural gas - the next diesel development step?

by Dr Tim White and Julian Edgar

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The massive improvements in power outputs of diesels that have occurred in recent years are the result of electronically controlled high pressure direct injection and intercooled turbos. These technologies have seen the power per litre of engine capacity rise to the extent that the use of the diesel engine in cars no longer incurs a performance penalty and in fact diesel-powered racing cars are now taking outright first places.

So what are the next moves going to be in diesel technology? One suggestion is that diesels should be dual fuelled. That is, rather than burning just diesel, they should also burn another fuel such as natural gas. Some diesel engines have been converted to run solely on natural gas while other technologies include using both diesel and natural gas, the two fuels injected near-simultaneously on each power stroke.

So what advantages are there in natural gas fuelled diesels and where is the technology right now?

Advantages

Burning natural gas in diesels has these advantages:

  • It burns more cleanly than diesel. Per unit energy, a reduction of the greenhouse gas Carbon Dioxide (CO2) of up to 20% is achievable when using natural gas instead of diesel.

  • For Australians, we have vast reserves (of mostly yet untapped) natural gas. At the present rate of consumption, proven reserves will last around 90 years whereas domestic oil reserves will last less than 40. Further, conversion of the Australian vehicle fleet to locally-available natural gas would greatly reduce the dependence of Australia on foreign petroleum.

  • As well as lower refining costs for natural gas when compared with diesel, natural gas is exempt from Australian federal government excise. In terms of price per mega-joule of energy, natural gas is less than half the price of diesel.

A dual fuel diesel engine can still run on just diesel, so a natural gas/diesel fuelled vehicle is not limited to being used only where there is the infrastructure for refuelling with natural gas.

Different Conversion Approaches

So that ignition begins shortly after the fuel enters the cylinder, fuels manufactured for use in compression ignition engines have a relatively high cetane number (CN).For instance, diesel oil has a CN of around 50. Gaseous fuels usually have low cetane numbers and so when used alone are not suitable for compression ignition. This is particularly true of LPG and natural gas which have CNs of around 10 and -10 respectively.

Since the alternative fuel cannot be ignited in the required time by compression ignition alone, an alternative means of igniting the charge must be implemented. There presently exists three main ways in which this may be achieved:

1. Converting the engine to Spark Ignition.

2. Retaining the diesel injection system and adding the gaseous fuel to the inlet manifold (called Conventional Dual Fuelling).

3. Retaining the diesel injection system and adding the gaseous fuel directly into the cylinder (called High-Pressure Direct Injection).

Each approach has advantages and disadvantages.

1. Conversion to Spark Ignition

When vehicles with existing compression ignition engines (such as trucks or buses) are converted to run on natural gas, they are often converted to operate solely on the gaseous fuel, ie. no diesel fuel is used. This is usually achieved by substituting a spark plug for the diesel injector in the engine’s cylinder head. The existing injection system is then replaced with a high-tension ignition system and the gaseous fuel is introduced through a mixer or carburettor before a throttle which fitted to the inlet manifold. Thus what was once a diesel engine effectively becomes a spark ignition engine. Conversion using these methods is able to be performed at relatively low cost. In more recent times, some performance improvements have been realised by the fitment of port injectors to replace the carburettor.

There are, however, many compromises which must be accepted when converting a compression ignition engine to spark ignition for the consumption of natural gas:

  • The range of the vehicle is short and it is limited to regions where the infrastructure for gas refuelling exists.

  • To ensure that ignition occurs, the fuel-air mixture in the cylinder must be near stoichiometric at all times. This means that the engine loses one of the fundamental advantages of compression ignition engines: the ability to operate on very lean mixtures without throttling, so minimising both emissions and pumping losses.

  • Typical compression ratios for spark ignition engines operating on natural gas are around 12:1. To achieve this much lower compression ratio, a spacer usually needs to be fitted between the block and head. Such a reduction in compression ratio results in the loss of thermal efficiency and torque, especially at low speed, both common advantages of compression ignition engines.

  • Studies of urban bus fleets showed that with conversion to natural gas operation with spark ignition, a fuel consumption increase of 25-30% is incurred. This negates much of the improved greenhouse gas performance of engines using natural gas.

2. Conventional Dual-Fuelling

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This diagram shows a schematic layout of an engine in which natural gas is introduced through the inlet manifold but ignition is achieved with the injection of a small amount of diesel, rather than with a spark plug. This technique, when applied to existing compression ignition engines, is known as Conventional Dual Fuelling.

This approach has both advantages and disadvantages.

An advantage is that the pilot diesel ignition is more effective than spark ignition since there are many individual ignition sources created. At least one ignition source at each jet from the diesel injector will occur - typically five or six in a modern engine - allowing for more complete and rapid combustion of the lean natural gas-air mixture than with a single spark plug. This in turn reduces emissions and the chance of knock.

Another advantage is that because ignition of the gas does not rely on a spark plug, near-stoichiometric mixtures at the time of ignition are not required, so the engine can continue to be un-throttled. Further, Conventional Dual Fuelling systems used for compression ignition conversion allow flexible variation from natural gas/diesel ratios. Up to about 90% natural gas can be used although operation on 100% diesel is available at any time, important when natural gas re-fuelling infrastructure systems are not currently widespread.

A disadvantage of the Conventional Dual Fuelling approach is a strong dependence on both the quantity and quality of the pilot diesel fuel, especially with increased proportions of gaseous fuel. Further, the amount of gas that can be substituted for diesel is limited by two factors:

  • With increasing gas proportions comes an increasing likelihood of engine knock

  • There is a minimum fuel amount that diesel injectors can flow so pilot fuel injection is normally kept high, in turn reducing total gas usage

Finally, the ignition of the pilot diesel injection is delayed by the presence of the gas, and emissions of oxides of nitrogen and unburned hydrocarbons can be higher than desirable.

3. High-Pressure Direct Injection

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Many of the disadvantages described above for conventional dual fuelled engines can be overcome by injecting both the natural gas and the pilot diesel fuel into the cylinder more or less simultaneously. With High Pressure Direct Injection, the natural gas is injected at an absolute pressure of around 20 MPa. A few degrees earlier, the diesel pilot is injected either through the same injector or another injector nearby. In this way, spark ignition-type knock is no longer an issue and therefore compression ratios need not be reduced. There is no need for throttling and the ignition delay of the diesel is not increased since the pilot fuel is injected into air only.

The direct-injection of natural gas with a liquid diesel fuel pilot has the following advantages:

  • The efficiency of the diesel cycle is retained

  • Spark ignition-type knock cannot occur if the gas injection is near simultaneous with the diesel

  • The engine requires no throttling

  • The cycle can operate as “lean burn” and requires no mixture ratio control

  • There will be negligible unburnt fuel in the exhaust

With co-injection, the difficulties are in achieving the right mixing of the fuels during their limited residence time in the spray zone.

Early work into High Pressure Direct Injection was carried-out on very large (usually ship) engines, particularly those for LNG tankers where the boil-off gas could be used to power the ship. It’s only in more recent times has High Pressure Direct Injection been realised with smaller engines.

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A number of researchers have achieved promising results. In 1998 Westport Innovations Inc announced an alliance with Cummins to incorporate High Pressure Direct Injection using liquid natural gas into a modern 4-stroke for stationary power generation. By 2002, a four-stroke, 15L Cummins ISX-400 (rated at 1,950 Nm) had been tested with the original injectors replaced with a new High Pressure Direct Injection system. Diesel was pumped to the rail at 25 MPa and gas was supplied to a parallel rail at the same pressure. The performance of this engine can be summarised as follows:

  • The pilot quantity remained roughly constant, independent from the load and so the percentage varied between 9 and 2.4% when the load increased from 20% to 100%.

  • Torque was maintained at diesel-only levels and composite cycle efficiency was maintained within 2.3% of the diesel-only baseline.

  • Oxides of nitrogen and particulate matter emissions reduced respectively to 45% and 70% of the baseline. Methane and carbon monoxide emissions were low for an NG engine operating with excess air.

  • CO2 emissions were approximately 20% lower than with full-diesel operation.

  • Field tests were carried out using 14 trucks which were part of the 38-truck waste-transfer fleet operating in San Francisco. The trucks accumulated 837,000 km, corresponding to about 15,000 hours of operation. Only one on-road failure was reported and that was when a truck ran out of fuel. On average, each truck consumed 94% NG and 6% diesel.

    More recently and in conjunction with Ford, Westport has begun developing an NG-fuelled CI engine for light-duty vehicles.

    Conclusion

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    In terms of emissions, running costs and geographical sourcing of fuel, burning natural gas in diesel engines has considerable advantages. Of the different approaches that can be taken, high pressure direct injection of the gas after a pilot injection of diesel fuel is likely to give the best results. This technology also dovetails well with the developments in high pressure injection being used across all road vehicle diesel engines.

    This article is based on a PhD thesis, Simultaneous Diesel and Natural Gas Injection for Dual-Fuelling Compression-Ignition Engines, completed by Tim White at the University of New South Wales in 2006.

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