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Diesel fuel are not strictly speaking an alternative fuel. However, in North America diesel (or compression ignition) engines are very uncommon in powering passenger vehicles. This is probably due to the poorly-built engines that North American automobile manufacturers introduced during the fuel crisis of the 1970's. Those engines were spark ignition (SI) engines converted to compression ignition (CI) rather than purpose-built compression ignition engines. The motoring public experienced failure-prone low performance engines and have stayed away ever since. Even the European manufacturers did not make a smooth running at that time. With North American consumers used to smooth running powerful gasoline engines, the smoke and diesel clatter did not help to convince these consumers to give these engines a chance. Times have changed. With the new millennium, automobile manufacturers (especially the Europeans) have been refining the compression ignition engine. There are not very many diesel-powered passenger cars available in North America. Volkswagen and Mercedes-Benz are two manufacturers that come to mind. For light trucks, most manufacturers offer a diesel option. BiodieselFor those people having diesel-powered vehicles, a new renewal fuel source is emerging. Biodiesel is made primarily either from animal or vegetable oils. In Canada, biodiesel is being made from soybean oil, canola oil, and mustard oil as those crops are produced in great abundance in its prairie provinces. Biodiesel is also made from animal fats and recycled cooking oil. Biodiesel can be used by itself (B100) but is more commonly blended 20% (B20) with petroleum-based oil. A great deal of research is going into the development of fuel from mustard oils in particular. A side benefit of this crop is that the mustard pulp can then be used as a natural pesticide. Dual FuelOne of the best applications for dual fuel is for stationary engines which are typically used for power generation, pumping, and running compressors. In this case, the engines are designed to consume natural gas with a small, pilot injection of diesel fuel to ignite this fuel mixture. Natural gas is used in this application because it has an extremely high resistance to self ignition from the high compression-ratios used in diesel engines. Like a conventional single-fuel diesel engine, there is no throttle valve and load control is done by adjusting the fuel mixture. These engines are among the most efficient in terms of kW produced per Joules consumed (hp produced per BTU consumed) because of the thermal efficiency of the diesel cycle. In the automotive world, there are kits available to convert your diesel truck engine to dual fuel operation. However, the important difference between automotive engines and stationary engines is that automotive engines rely on propane as the gaseous fuel and this is simply because propane's far better energy density. This practical reality does not make up for propane's one inferiority compared with natural gas, which is propane's resistance to self-ignition. Propane's Anti-Knock Index or Octane Number is much lower than natural gas's (104 compared with 120+) which limits the static compression ratio to around 12:1. A typical diesel engine has much higher compression ratio than this so a propane-fuelled diesel engine will knock severely with sufficient amounts propane. Franz Hofmann has written an excellent FAQ on the Eng-Tips forum and I highly recommend that you read it you are considering a dual-fuel conversion for your diesel engine. He further recommends against using LPG on engines with a static compression ratio higher than 16.5:1 and is very much against driver-controlled LPG metering. There is great deal of misinformation on the internet and, if your engine is damaged by a dual fuel system, you will be on your own. In reading through the FAQ of one dual fuel system, I came across the following statement:
This statement clearly acknowledges that their system will violate your manufacturer's warranty. Now that these systems are in existence, your dealer will be looking for signs of a dual fuel system installation when you bring it in for service on an engine failure. Another bit of misinformation is the idea that LPG is a catalyst to improve the combustion diesel fuel. By definition, a catalyst is a substance that causes or speeds up a chemical reaction by the presence of the catalytic substance that in itself remains unchanged. In a diesel engine, for practical purposes, all fuel is already consumed. With a dual-fuel system, both the LPG fuel and the diesel fuel will be consumed so there is no way that LPG could possibly be considered a catalyst. Another dual fuel system web site states that:
It is interesting that they would say this because, due to its high Octane Number, LPG is a fuel better suited the Otto cycle which is considered to be a constant volume process. The Otto Cycle is one in which the fuel mixture reacts so quickly that the piston moves very little during the combustion process. The Diesel cycle is considered to be a constant pressure cycle which means that, as fuel is being sprayed into the combustion chamber, the fuel combusts about as fast as it is sprayed-in which results in a much slower rise in cylinder pressure. Also, the additional fuel can only increase serve to increase combustion temperatures because the system introduces more energy to and reduces the diluting air from the combustion chamber. Because the thermal efficiency of the engine has not increased, there is no way for there to be a huge increase in fuel mileage. You may be buying less diesel fuel but now you are buying more LPG which contains less energy per unit of volume. Only if your local propane price is significantly lower than diesel will you be reducing your driving costs. Obviously, an engine will produce more power when it consumes more energy and this requires putting more fuel mixture into a cylinder per cycle. In practical terms, diesel engines are designed to operate with excess air and adding more fuel will make combustion hotter, thereby making more power from the resulting higher temperatures and pressures. It is possible to increase the power output of a diesel engine by adding small amounts of propane to the air stream. Too much propane will cause it to self-destruct. Because a gaseous fuel will displace some of the combustion air (as it would in a gasoline engine as well), a turbocharger is the best way to obtain the full benefits from the addition of a gaseous fuel. Ideally, the amount of propane added to the fuel mixture must be carefully controlled by an electronic fuel metering system, which takes into account many engine variables. Stay away from any driver-adjustable system. |
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