The Cummins A3.4-125 (rated 93 kW at 3600 rpm) has been developed to meet 1991 US and California light-heavy duty emission standards, replacing the Cummins 6AT3.4 (formerly Onan L634T-A). Compliance with the stringent particulate standard has been achieved by redesigning the combustion chamber, a systematic oil control program, and charge air cooling. The Ricardo Comet combustion chamber was modified to a downstream glowplug configuration. Oil control efforts addressed all sources of oil derived particulate. With charge air cooling, NOx emissions were reduced while improving fuel economy, torque output, altitude capability, and engine durability.THE CUMMINS A3.4-125 is an evolutionary development of the 1988-90 6AT3.4 engine. The development was driven primarily by 1991 US and California light-heavy duty emission standards, but also was the result of a policy of continuous product improvement.The Cummins A Series diesel engine family was conceived as the Onan L Series (1*). That design began with a clean sheet of paper without tooling constraints. The designers were thus free to minimize package size and optimize application flexibility, structural characteristics, and service ability.The A Series family includes naturally aspirated in-line engines of 3, 4, and 6 cylinders. The 6 cylinder engine is also available in turbocharged and turbocharged/charge cooled form. All models are four-stroke and share a common bore of 89 mm and stroke of 92 mm. Cylinder displacement is 0.572 liters; the A3.4-125 displaces 3.434 liters.The engine was developed with an indirect injection combustion system, utilizing the Ricardo Comet V design. This combustion system was selected for its smooth combustion, low emissions potential, low noise, and minimal torsional vibration characteristics.The A3.4-125 features seven main crankshaft and seven camshaft bearings, a helical geartrain for precise cam and injection pump timing, an integral oil cooler with no external lines, a Bosch VE6 injection pump (with speed and load dependent timing control, boost-dependent fuel delivery, and cold engine timing advance), and a low-inertia Garrett TB2205 wastegated turbocharger (calibrated for good low speed driveablity and high speed cylinder pressure control). Also featured are hydraulic valve lifters for low maintenance, one-side service ability, and glow plug assisted starting capability to -29°C (-20°F).In the course of the 1991 emissions development process, the low emissions potential of the IDI combustion system has been combined with a detailed oil control program and charge air cooling to meet US and California light-heavy duty emission standards while improving fuel economy and torque output.The A3.4-125 is designed for on-highway emissions certified applications of 3850 to 8825 kg (8,500 to 19,500 lbs) gross vehicle weight. The primary use is in Class 2, 3, 4, and 5 commercial vehicles such as package and delivery vans.The engine offers low noise (due in part to the IDI Ricardo combustion system), low vibration (also due to the combustion system but also to the inherent characteristics of an in-line six cylinder engine) and excellent driveability. The engine's wide operating range (1600 rpm peak torque and 3600 rpm rated speed) results in driveability similar to 5 liter (300 cubic inch) in-line six cylinder gasoline engines. The A3.4-125 is designed for easy maintenance and includes many heavy duty design features for a BIO durability target of 177,000 km (110,000 miles) and and a B50 of 258,000 km (160,000 miles).