On August 18, 1992, Tecogen Inc patented a "Method for Converting a Diesel Engine to a Natural Gas Fueled Engine":
A method for converting a diesel engine to a natural gas fueled engine, the method comprising the steps of providing new cam profiles on a cam shaft portion of the diesel engine, the new cam profiles being operative to close intake valves of cylinder heads of the engine during intake strokes, operative to close exhaust valves of the cylinder heads, and operative no earlier than the exhaust valves closures to open associated intake valves, modifying portions of the cylinder heads to adapt the cylinder heads to accept spark plugs, fixing spark plugs in the cylinder heads, providing means for conveying natural gas fuel to the intake valves, providing a blower for supplying air under pressure to the intake valves, and providing an after-cooler for cooling the air supplied by the blower prior to the air reaching the intake valves.
(...) It therefore would be beneficial to have available a method by which a diesel engine could be converted to run on natural gas, without the requirement of substantial modification or replacement of the cylinder heads and/ or pistons, and which would provide for proper timing of exhaust valve and inlet valve operation.
(...) It is, therefore, an object of the invention to provide a method for converting a four-stroke diesel engine to a spark ignition natural gas engine without replacement of, or substantial modification of, the cylinder head and/or pistons. A further object of the invention is to provide a method for converting a diesel engine to a natural gas engine, with modification of timing of operation of exhaust valves and inlet valves such as to accommodate the octane of the natural gas and to permit use of a pre-mixed fuel/ air metering system in a turbo charged or supercharged engine, to provide improved mixing of fuel and air, with consequent improved combustion and emissions. A still further object is to provide such a method resulting in a natural gas engine having substantially the same power output as the diesel engine, and having variable spark timing for high performance at various speeds and torques. A still further object is to provide such a method as is applicable to diesel engines of all sizes.
(...) With the above and other objects in view, as will hereinafter appear, a feature of the present invention is the provision of a method for converting a four—stroke cycle diesel engine to a natural gas fueled engine, the method comprising the steps of providing new cam profiles on a camshaft portion of the diesel engine, the new cam profiles being operative to close intake valves of cylinder heads of the engine during intake strokes, operative to close exhaust valves of the cylinder heads, and operative no earlier than the exhaust valves closure to open associated intake valves, modifying portions of the cylinder heads to adapt the cylinder heads to accept spark plugs, fixing spark plugs in the cylinder heads, providing means for conveying natural gas fuel to the intake valves, providing a blower for supplying air under pressure to the intake valves, and providing an after—cooler for cooling the air supplied by the blower prior to the air reaching the intake valves.
(...) The above method provides a number of advantages including, most importantly, a number a steps rendered unnecessary. For example, in the above-described procedure it is not necessary to undertake modification of the piston, engine block and crank shaft. Of the major engine components, only a minor modification to the cylinder head is required to adapt the cylinder head to receive spark plugs. Changing the cam profiles of the camshaft does not affect engine durability or the dynamic characteristics of the rotating components of the engine. Among performance advantages are a low effective compression ratio and large expansion ratio providing a high thermal efficiency, low exhaust gas temperature, low exhaust emissions, and retention of the diesel power output. The variable spark timing contributed by the ignition system and spark plug optimizes performance and contributes to minimization of emissions. Further, the procedure described above is applicable to diesel engines of all sizes.
On April 9, 1991, the ARB made the following announcement (read source):
The California Air Resource Board (ARB) announced today that it has certified the first alernative fueled engine for use in California. a compressed natural gas—powered engine that will be used in school buses.
That certification paves the way for the first mass production of alternative fueled vehicles in the state, a step forward from from the experimental permits under which other, individual buses have been built.
The TecoDrive 7000 compressed natural gas-powered engine, built by Tecogen Inc. of Walthem, Massachussetts, will be used in then new school buses by some Los Angeles, San Diego and San Francisco Bay Area school districts, beginning September 1991.
James Boyd, ARB executive officer said, "The Tecogen engine is the type of alternative fuel powered engine envisioned by the ARB when it adopted low emission vehicle standards during its public hearings held here in Los Angeles las September.
"These natural gas powered engines show that alternative fueled vehicles are not dreams of the future and not just experimental, but are available now to help clean California's air, he added."
Last September, the ARB adopted "ultra clean" standards for California vehicles that are 50 to 85 percent less polluting than the strictest standards already adopted by ARB. The new standards will produce about 200,000 of the new "ultra-clean" cars, ten percent of the state's annual new car fleet, in 1994. Those standards have caused vehicle manufacturers and energy providers to design engines and fuel that act as a single system for the first time, and has forced them to consider alternative fuels as much of meeting those standards.
According to ARB data, the compressed natural gas (CNG) powered buses, which seat 66 students, emit less than one-third of the smog-forming hydrocarbon and nitrogen oxide emissions allowed by the ARB's tough emission limits. In addition, it requires three of these buses to match the nitrogen oxide output of one comparable gasoline powered engine.
(...) California, through the ARB, has certified conversion kits for existing gas or diesel powered engines. However, the Tecogen engine is the first production line model to be certified.
Bibliography :
- Executive order A-300-1 - ARB - State of California - 02/27/1991
- Executive order A-300-2 - ARB - State of California - 04/09/1991
- Executive order A-300-3 - ARB - State of California - 12/18/1991
- Executive order A-300-4 - ARB - State of California - 09/13/1993
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