PART 1: It’s the burning question: which fuels will power future internal-combustion engines?
(See part 2 here)
MOST of the energy we use on a daily basis comes from the sun. This energy is stored in various forms and humans depend on it for all the aspects in our daily lives, including transport. With finite fossil-fuel reserves, manufacturers are more focused than ever on finding alternative sources of energy. In the first instalment of a two-part series, we look at the types of fuels available for use in the internal-combustion engine.
Petrol
Energy density (Megajoules/litre): approx. 34
Source: mostly crude oil
Pros: high energy content; developed infrastructure
Cons: fossil fuel; emissions (including CO2)
Petrol is a collective term for a vast range of fuels available at fuelling stations and which are used to operate spark-ignition engines. The research octane number (RON) denotes the resistance to knock (or pre-ignition). The higher the number, the more resistance the fuel has to this destructive phenomenon. A high RON fuel does not contain more energy but allows engines to run higher compression ratios (or boost pressures), which increase outputs. The spark timing in modern engines is advanced to a point just before knock occurs to increase efficiency and power output. Other important characteristics of petrol are volatility, density, vapour pressure and the vapour/liquid ratio. Oil companies add different additives to petrol in the form of detergents, corrosion inhibitors and oxidation stabilisers.
Diesel
Energy density (MJ/litre): approx. 37
Source: mostly crude oil
Pros: high energy content; developed infrastructure
Cons: fossil fuel; emissions (including CO2)
The cetane number denotes the ignition quality of diesel (the higher the number, the greater the tendency to ignite) and can be seen as an opposite of the octane number of petrol. Diesel is prone to cold-weather waxing, which can block a fuel filter in temperatures below zero degrees Celsius. The filtration limit is used to determine the resistance to waxing. The sulphur content is important because it degrades engine-oil performance and can contaminate the exhaust after-treatment system of a modern diesel engine. Other important characteristics are flash point, density, viscosity, lubricity, carbon-deposit index and water content. Additives are also added to diesel to improve the lubricity, flow in cold climates, cleaning and corrosion properties and anti-foaming characteristics.
Biodiesel
Energy density (MJ/litre): approx. 33
Source: vegetable or animal oils trans-esterified with methanol
Pros: high energy content, renewable
Cons: biofuel, emissions (including CO2)
Biodiesel has a lower energy density than regular diesel but greater lubrication properties, which may be beneficial to fuel-injection hardware. For automotive use, biodiesel is usually mixed with regular diesel at a fixed ratio. The letter B denotes biodiesel and the number following it refers to the mix percentage; for example, B20 is diesel with 20 per cent biodiesel content. Some manufacturers do not allow more than five per cent biodiesel content to avoid impacting the warranty of the vehicle.
Ethanol
Energy density (MJ/litre): approx. 24
Source: mostly corn, sugar cane, rice or wheat
Pros: octane enhancer, renewable
Cons: biofuel, low energy content, hygroscopic, corrosive,
emissions (including CO2)
Ethanol is an alcohol-based fuel that can be produced from various crops and is therefore classified as a biofuel. As the specific energy content is relatively low compared with petrol, ethanol is used mostly as an additive to enhance the octane rating of conventional petrol. E denotes ethanol and the number after the percentage content, for example E85, has 85 per cent ethanol and 15 per cent petrol. Some engines are developed to run on pure ethanol, but the fuel consumption is considerably higher than that of the petrol equivalent owing to the lower energy content. Higher compression ratios (or boost values in turbo engines) can be employed because of the knock resistance of ethanol. Negatives of ethanol include the fact that it is hygroscopic (absorbs water vapour from the atmosphere) and is corrosive to aluminium components.
Methanol
Energy density (MJ/litre): approx. 16
Source: mostly natural gas
Pros: octane enhancer
Cons: Low energy content, hygroscopic, corrosive, toxic, emissions (incl. CO2)
Methanol is an alcohol fuel like ethanol, but is used less in flexi-fuel vehicles owing to its lower energy content and toxicity. The fuel consumption of a vehicle running on methanol is roughly twice as high compared with one operating on petrol because of the low energy content and a stoichiometric air-fuel ratio of approximately 6,5:1. Methanol is used mostly as an octane-enhancing additive in petrol (and on the racing scene).
Liquefied petroleum gas (LPG)
Energy density (MJ/litre): approx. 26
Source: mostly crude oil
Pros: high octane number; non-corrosive
Cons: fossil fuel, range due to small-capacity storage tank, emissions (incl. CO2)
LPG is essentially a mixture between propane and butane gas, and is liquefied under pressure. It is a fossil fuel and a by-product of the crude-oil refining process. In an internal-combustion engine, it burns cleaner than petrol because vaporisation is better and there are fewer particulate emissions. Although the calorific specific energy of the gas is high at
46,1 MJ/kg, the energy density in liquid form is much less than that of petrol. Therefore, more liquefied gas per kilometre is used to attain similar performance.
Synthetic fuels
Laboratories have proven that it is possible to produce ethanol and diesel synthetically using only salt water, sunlight, CO2 and special algae (the latter’s DNA undergoes modification to allow for the creation of fuel that conforms to automotive specification for fuel that can be used in modern engines). The challenge now is to set up a high-volume plant to produce fuels at a competitive market price. Two companies that specialise in this field are Joule Unlimited, in conjunction with Audi (see the special report on page 116), and Sapphire Energy. The biggest advantage of synthetic fuels is that CO2 is part of the process, making the fuel carbon-neutral (burning the fuel releases the CO2 used for production back into the atmosphere) and renewable.
Hydrogen
Energy density (MJ/litre): approx. 4,5 at 700 bar
Source: water
Pros: environmentally friendly
Cons: infrastructure/storage problems, producing hydrogen is complex, low energy density on a volumetric basis results in limited range
Hydrogen does not occur naturally on Earth, so it is essentially not an energy source but only an energy carrier. Therefore, other energies are used to produce hydrogen, a process that has the potential to be very inefficient. The ideal combustion of hydrogen and oxygen will produce only water as a by-product. Although hydrogen has a very high specific energy density (120 MJ/kg), it has a low volumetric energy density compared with petrol. This means that an elevated storage pressure is needed to give a vehicle with an internal-combustion engine that runs on hydrogen adequate range. For example, a litre of petrol will have the same energy content as roughly 7,5 litres of liquefied hydrogen at 700 bar.
Compressed natural gas (CNG)
nergy density (MJ/litre): approx. 9 at 250 bar
Source: exists in natural form
Pros: High octane rating (130+), renewable
Cons: low energy density per unit volume, limited range due to small capacity
of pressurised storage tank, greenhouse gas, emissions (including CO2)
The primary component of natural gas is methane. It is an alternative to fossil fuels because it can be produced by fermentation of organic-waste material. The high octane rating allows for the use of high compression ratios and higher boost values in turbo engines, but without the negative effects of knock. Its specific energy is unfortunately on the low side, which results in limited range in a CNG-only vehicle. Therefore, most CNG automotive applications are flex-fuel vehicles, in which this fuel can also run on petrol (to increase the range).
Summary
The assumption that fossil resources will last forever is short-sighted. Biofuels appear to be the perfect solution, but the ethical concerns of using food for fuel renders this solution less than ideal. Meanwhile, there are too many obstacles regarding the production and storage of hydrogen to make it a feasible alternative. The best chance the internal-combustion engine has for survival is to use natural gas and synthetically produced fuels.
Read about alternative energy here, in part 2 (click).