1980, without a catalytic converter. The closed-loop version of ELB used an oxygen sensor and a feedback carburetor, and was phased into production as emissions regulations grew more stringent starting in 1981, but open-loop ELB was used as late as 1990 in markets with lax emissions regulations, on vehicles such as the Mexican Chrysler Spirit. The spark control and engine parameter sensing and transduction strategies introduced with ELB remained in use through 1995 on Chrysler vehicles equipped with throttle-body fuel injection. Heavy-duty gas engines Lean-burn concepts are often used for the design of heavy-duty natural gas, biogas, and liquefied petroleum gas (LPG) fuelled engines. These engines can either be full-time lean-burn, where the engine runs with a weak air–fuel mixture regardless of load and engine speed, or part-time lean-burn (also known as "lean mix" or "mixed lean"), where the engine runs lean only during low load and at high engine speeds, reverting to a stoichiometric air–fuel mixture in other cases. Heavy-duty lean-burn gas engines admit twice as much air as theoretically needed for complete combustion into the combustion chambers. The extremely weak air–fuel mixtures lead to lower combustion temperatures and therefore lower NOx formation. While lean-burn gas engines offer higher theoretical thermal efficiencies, transient response and performance may be compromised in certain situations. However, advances in fuel control and closed loop technology by companies like North American Repower have led to production of modern CARB certified lean burn heavy duty engines for use in commercial vehicle fleets. Lean-burn gas engines are almost always turbocharged, resulting in high power and torque figures not achievable with stoichiometric engines due to high combustion temperatures. Heavy duty gas engines may employ precombustion chambers in the cylinder head. A lean gas and air mixture is first highly compressed in the main chamber by the piston. A much richer, though much lesser volume gas/air mixture is introduced to the precombustion chamber and ignited by spark plug. The flame front spreads to the lean gas air mixture in the cylinder. This two stage lean-burn combustion produces low NOx and no particulate emissions. Thermal efficiency is better as higher compression ratios are achieved. Manufacturers of heavy-duty lean-burn gas engines include MTU, Cummins, Caterpillar, MWM, GE Jenbacher, MAN Diesel & Turbo, Wärtsilä, Mitsubishi Heavy Industries, Dresser-Rand Guascor, Waukesha Engine and Rolls-Royce Holdings. Honda lean-burn systems One of the newest lean-burn technologies available in automobiles currently in production uses very