Crude oils and liquid fuels have been used for powering internal combustion engines since the time of their invention. This is mainly due to their high energy density, higher thermal efficiency than solid fuels and ease of transportation and handling. With evolutions in engine development and the demand for more power, efficiency and drivability, petrol and diesel have been mainly used by most car manufacturers and high-performance racing teams. The shortage of crude oil resources and changing of international policies towards emissions and air pollution, make the future of automotive fuel uncertain. Motor vehicle emissions currently are estimated to contribute about 14% global Carbon Dioxide emissions, 50-60% of Carbon monoxide, Hydrocarbons and lead emissions, 30% of Nitrogen Oxides emissions and 10-20% of particulate matter emissions. (3) Due to this, countries in the world are trying to create new alternate fuels which might substitute for petrol or diesel fuel. Effective use of alternate fuel makes stable fuel supply possible. (1) Alternate fuel is expected to satisfy the following conditions.
There is abundant resources to produce it.
Mass production is possible.
It can maintain a stable supply.
Energy density is comparatively high and easy to handle.(2)
Although, there is this current drawback where the current cost for most alternate fuels is higher than the standard fuels used due to the lower demand and quantities in which they are used. If the use of alternate source fuels becomes more commonplace then this drawback will eventually not be of significance.
The major cause of concern for the standard petroleum fuels used today is the amount of emissions they cause. There have been various studies conducted to study the effects of NOx emissions, Hydrocarbon and Carbon monoxide emissions and particulate matter emissions conducted on petrol and diesel engines, used on light duty to heavy duty ground vehicles from various regions and regulatory norms. Karlsson, H., Gåsste, J., and Åsman, P. (2008) conducted a study on Regulated and Non-regulated emissions by comparing petrol and biogas based engines and engines that used alcohol rich fuel mixtures that followed the NEDC and Artemis cycles. The emission tests were carried out in different driving conditions and climatic conditions, i.e. in both summer and winter conditions where the fuel mixture was altered according to the weather. What was found was that the E85 ethanol and petrol mixture (85% ethanol) fuel led to reductions in NOx, 1, 3-butadiene, benzene, toluene and other Hydrocarbons emissions but increases in aldehyde emissions during the tests. The overall energy consumption of ethanol fuels was higher in comparison to petrol based fuels.
For diesel fuelled engines, Hammerle, R., Ketcher, D., Horrocks, R., Lepperhoff, G. et al., (1994) measured the regulated and non- regulated emissions from 6, well- maintained, 1992/93 model diesel vehicles to study their effect on urban ozone layer impact and impact from their emissions based on both the US FTP and European MVEG cycles. Conclusions from their studies showed that though diesel engines showed lesser Carbon monoxide and lower- hydrocarbon emissions, NOx emissions, higher- hydrocarbons, aldehyde emissions and particulate matter emissions were much higher for these engines when compared to petrol engines of the same era. These emissions have a significant impact on the surrounding environment especially the ozone layer above the concerned urban area.
We will be looking at the fluid based alternative fuels that can be used to substitute the traditional petrol and diesel in an internal combustion engine (Pulkrabek 150):
Alcohol as an alternate source of fuel is not from a new source. During the invention of the internal combustion engine, manufacturers initially used alcohol (after charcoal) as their source of fuel, mainly because it was easily available and cheap. Alcohol can be obtained from several sources, both natural and manufactured. Methanol and Ethanol are the most preferred and most commonly used alcohol fuels for internal combustion engines. Pure methanol and ethanol and their mixtures with petrol and diesel are under major research and development nowadays. The most common alcohol- petrol mixtures used are M10, M85, E15, and E85 where the character tells us the alcohol that has been mixed (methanol or ethanol) and the number following the character tell us the percentage of alcohol in the mixture (5). There is also some research going into the use of a tri-fuel mixture using both ethanol and methanol. Following is the list of some of the advantages and disadvantages of using alcohol as an alternate source.
1. Alcohol has a very high octane number and can hence increase the overall efficiency of the engine as it can be used at higher compression ratios. It also has a higher flame speed and anti-knock index.
2. It has lesser overall emissions in comparison with a petrol or diesel engine.
3. Higher evaporative cooling which increases the volumetric efficiency of the engine.
4. Lower sulphur content in the fuel.
5. Sources for this fuel are highly abundant.
1. The fuel has an overall lesser energy content which means almost twice as much alcohol needs to be burnt to give the same energy.
2. Aldehyde emissions in the exhaust are very high.
3. Corrosive nature of alcohol, especially methanol on metals, can be a bothersome factor in the design and manufacture of the engine and other parts that could be affected.
4. Poor ignition characteristics and cold-start issues.
5. The danger of flammability in storage tank due to the low vapour pressure of alcohol. Low pressure could also lead to vapour lock in the fuel delivery system.
6. Alcohol, due to its high reactivity also tends to combine with any moisture present. This could be an issue when alcohol is used as part of a mixture and it tends to separate itself out from it. This could lead to improper combustion in the combustion chamber which could lead to more harmful emissions. Hence alcohol needs to be stored and maintained very carefully.
There are several pieces of research where alcohol is also used in Combustion ignition engine, i.e. in a mixture with diesel. Adelman, H., (1979) tried to study the effects of mixing a high octane fuel like alcohol with diesel and ran the mixture in a combustion ignition engine. The study concluded by finding that no major engine and fuel system modifications are required for the diesel- alcohol mixture to work in a combustion ignition engine. This is because fumigation can be done by injection of a diesel alcohol emulsion or separately injecting diesel and alcohol in the required ratio. There was no apparent loss in efficiency or power in the engine by using the mixture or increase in emissions. There was, however, a decrease in NOx and Particulate matter emissions.
Hydrogen has also been considered as a major replacement for conventional fuels. It was used in the past for sea-ships but the usage was stopped because of its high flammability. Kukkonen, C., 1981, conducted a study on the feasibility of replacing petroleum-based fuels with hydrogen in Internal combustion engines. The feasibility was assessed considering the production of the fuel, onboard storage, distribution and suitability to be used as a fuel. The study concluded that although hydrogen has several advantages as a fuel, it should rank last as a replacement fuel for internal combustion engines (9). The advantages and disadvantages of using hydrogen as a fuel have been listed below. (Pulkrabek 157)
1. As there is no carbon in the fuel, there will be only water vapour and nitrogen at the exhaust of such an engine. Hence, very low emissions. Fuel leakage to the environment is also not an issue.
2. Easily available and has several sources for production.
3. High energy content per volume when stored and used as a liquid.
1. Heavy and bulky storage system, both onboard as well as in service centre. The energy spent to store hydrogen is very high, making the overall system efficiency very low.
2. Highly detonatable. It is also very risky to store and fuel leaks could lead to fatal accidents.
3. As it is a gaseous fuel, engine volumetric efficiency will be very low.
4. High NOx emissions because of higher flame temperature. There is also a risk of pre-ignition in the conventional design of the internal combustion engine.
Only one automotive manufacturer, Mazda, who have used the rotary Wankel engine in the past have adapted to using hydrogen as a fuel in their engine. It was reasoned that since in the design of this engine, the intake and the exhaust are at opposite sides to each other, the chances of pre-ignition of the gas are minimal.
Natural Gas – Methane
Natural gas is a mixture of components, mainly Methane (60-98%) with traces of hydrocarbons, carbon dioxide and nitrogen. It is usually stored as compressed natural gas at very high pressures and as liquid natural gas at slightly lower pressure but much lower temperature. (Pulkrabek 157) We can commonly find CNG used in third world countries like India and Pakistan as an alternative fuel for taxis and buses as it can be very commonly found in most rural and urban areas. Several tests and research are being conducted on Natural Gas Vehicles by several governments of the world today (11, 12). Advantages and Disadvantages of CNG are:
1. High octane number (120) so can be used in high compression ratio engines.
2. Highly abundant and cheap source of fuel.
3. Low engine emissions with lesser aldehyde emissions than methanol.
1. Low energy density and low volumetric efficiency resulting in lower engine performance.
2. There is a need for a large pressurised storage tank which is also a cause for safety concerns. The range of the vehicle depending on the size of the tank is also very low.
Methane is also used in a mixture with diesel in compression ignition engines, where methane is supplied as a gas through the fuel system and diesel is only used to ignite the gas. This results in a very clean running engine. Methane can also be substituted as a fuel in other power plants.
Kalam, M., Masjuki, H., Mahlia, T., Fuad, M. et al., (2009) have conducted a recent study and developed a system where Natural gas can be directly injected into the combustion chamber. They did this by modifying a Multi-port direct injection petrol engine setup by substituting the liquid fuel injectors with high-pressure gas injectors and varying the working compression ratio of the engine. The results of their study showed that the overall engine power produced by the CNG-DI engine was higher than the conventional engine, the BSFC of the CNG-DI engine, the overall NOx, hydrocarbon and carbon monoxide emissions were all lower than the conventional engine.
There are several other sources of fuels which can be considered. Electricity is a major source but it has not been discussed in this review as this review involves replacement fluid fuels for the conventional ones we have today. Electricity is, however, the most promising source of fuel of them all. Researchers and Manufacturers have also looked into using Propane, vegetable oils from barley, soy-beans, rapeseed, etc., called as Bio-fuels (Pulkrabek 151). Although these are more easily available and address the issue of shortage of petroleum, they do not reduce the overall pollution that their combustion causes.
Although, for most of the 20th century, the two main fuels that have been used in internal combustion engines have been petroleum fuels, with increasing air pollution, emissions and climate change issues and an overall petroleum shortage looming on the horizon, major research and development programs are being conducted throughout the world to find suitable alternative fuels to supply engine needs for the coming decades. In the foreseeable future, promising alternative sources of fuel like alcohol, electricity or hydrogen-based fuels will take over the automotive industry and we as automotive engineers must prepare for this change and also help further develop this change to a cleaner future.