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
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
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.
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
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.
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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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