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Conventional diesel or gasoline vehicles can be converted to the use of alternative fuels. Vehicles can be converted to run on a variety of fuels such as methanol, ethanol and biodiesel . However, we will focus mostly on conversions to compressed natural gas (CNG) and propane (LPG) ( 1 ) since they are the most common fuels used for this purpose.

Conversion of a particular vehicle to an alternative fuel can be done by altering the existing (old) engine system so that it is able to run on the alternative fuel, or by exchanging the old engine for an entirely new one optimized for the use of the alternative fuel. This latter form of conversion is referred to as "repowering" ( 2 ). Unless otherwise stated, in this section the ( w1 ) - form of conversion using the old engine is being discussed.

The most common types of buses to convert are city buses, because they normally operate in city areas where access to fuel can be arranged. There are two principal ways to make an existing (in-use) bus capable of operating on gaseous fuels.

The first alternative is to convert the engine to the Otto cycle by mounting spark plugs and a new engine control system. This can be done either by replacement of the engine ("re power") or modification of the engine. Gas tanks and related fittings must also be installed.

The vehicle becomes a "dedicated" CNG (LPG) vehicle and its performance (including emissions and fuel consumption) will depend on the quality of the conversion.
The other alternative is to install an "aftermarket" conversion kit. The engine will in this case remain working under the Diesel cycle.
The conversion kit can be either of the "fumigation" type, where the gas and diesel are mixed before being injected into the combustion chamber, or the "pilot injection" type, where diesel injection is retained corresponding to the fuel required to maintain idle conditions, and the gas/air mixture is designed to respond to all higher power requirements.

In an aftermarket conversion of a gasoline fueled vehicle, a conversion kit is added to the vehicle so that it can operate on an alternative fuel.

"Fumigation" or "Pilot Injection conversion conversion equipment generally consists of the following ( 1 ):
fuel tanks, fuel lines, a pressure regulator, and a mixer or carburetor to mix the fuel with incoming air.

Most conversion kits also include an electronic system to control the fuel/air mixture for optimal emissions performance. All original emissions control equipment should remain on the vehicle ( 1 ).

In converting a conventional diesel engine, additional steps must be taken to ignite the fuel as CNG has a very high octane value but a very low cetane value. Therefore "fumigation" or "pilot injection" converted diesel engines rely on some diesel combustion to initiate combustion of the CNG. For the "dedicated" conversion, this is achieved by installing spark plugs.

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In the sections on CNG Buses and LPG Buses, there is information on the potential for emission reductions by OEM, i.e. new buses or new engines. Most emission tests are done on OEM vehicles. The exact emission level of a converted vehicle depends very much on the type and quality of conversion. Thus emission levels of "primitive conversion systems can be significantly higher than those of a diesel engine" ( 2 ).

Experience show that it is very hard to find success stories for converted engines related to emission performance and general performance of the vehicle.

In their study on the introduction of CNG buses in Delhi ( 2 ), a team of experts stated that "retrofitting is made on old used engines which show a certain degree of wear and tear, and it is doubtful that the conversion agencies will repair the used engine according to the manufacturer's specification. So, from this standpoint, retrofitted in-use engines (unless produced to a very high standard of quality) are likely to have higher emissions than OEM engines."

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Conversions can be of variable design and quality. Improper installations or tuning of the conversion system can lead to excessive power loss. Power and performance can be optimized by adjusting the air/fuel mixture, compression ratio and ignition timing ( 1 ).

When converting or re power vehicles with CNG engines it is essential to address not only emission performance, but also safety aspects of the vehicle. Since leaking gas might cause serious accidents, special concern must be given to the gas installation on the vehicle.

Dursbeck, Erlandsson and Weaver, in their study on the status of implementation of CNG as a fuel for urban buses in Delhi ( 2 ), reported that "no provisions have been defined in the existing regulations for the inspection of in-use buses after conversion to CNG. Some conversions evaluated by us showed seriously deficient installations. In order to guarantee compliance of the converted buses with the specifications of the type approved vehicle, we highly recommend that each and every bus be inspected before allowing them to enter service. This inspection can be seen as equivalent to the foreseen conformity of production (COP) inspections for OEM CNG buses. Furthermore, this complies with the respective regulations in numerous other countries."

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The investment payback of a conversion depends on how many miles the vehicle travels in a year, the fuel economy, the initial cost of the conversion, operational costs and the vehicle resale value. The conversion equipment adds weight to the vehicle and, therefore, might reduce the payload (number of passengers) and slightly increases fuel consumption. If the alternative fuel is less expensive than the conventional fuel, however, the cost of the conversion may be paid back over time ( 1 ).

It should be noted that, as with the emission reductions, conversion costs may vary according to type and quality. The conversion of a diesel engine to dedicated CNG is considerably more expensive than installing an aftermarket "kit" as described above.
However, conversions are in most cases far less expensive than purchasing new OEM vehicles.
The Australian Bus Industry Confederation ( w2 ) gives some cost estimates for conversion of diesel buses to CNG. The IEA estimates the cost of conversions from diesel trucks to be in the same order as the cost of second hand buses in developing countries, i.e. between US$ 10,000 and 40,000 ( 3 ). For a comparison of vehicle technology cost estimates see also the Cleaner Buses Tabular Overview.

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Up to the first half of the 1990s, most alternative fuel vehicles (AFVs) were conversions of gasoline or diesel vehicles, and among these, LPG was the most commonly used fuel. Today, however, this situation has changed, and conversions play a decreasing role on the market as the range of available OEM AFVs increases ( w1 ). At the same time, natural gas (CNG and LNG) is gaining importance on the alternative fuel market. Of all alternative fuel types, propane (LPG) has the largest percentage of converted vehicles ( w1 ). This is due to a large number of converted passenger cars with gasoline engines.

For information on experiences with conversions to CNG, see also the case description of CNG Buses in Delhi.

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As with OEM vehicles, obstacles to the widespread use of vehicles converted to the use of alternative fuels include the incomplete fueling infrastructure (gas pipelines, refueling gas stations), as well as demand for additional knowledge about handling, inspection and maintenance,repair methods, safety issues etc. (see also the CNG Bus and LPG Bus pages). Furthermore, quality checks are of utmost importance, as conversions may increase emissions if carried out inadequately. In general, in order to assure that the desired emissions reductions are actually obtained, replacement of existing diesel bus engines with new alternative fuel engines is the preferred option in spite of the increased cost relative to conversions.

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