RIGHT-SIZING RATHER THAN DOWNSIZING

Reducing the number of cylinders of commercial engines

1. October 2015 | Engineering Service

Downsizing is an effective measure to reduce fuel consumption in passenger car engines. However, this trend cannot be readily transferred from passenger cars to commercial vehicles. Here the focus is on “rightsizing” with the aim to better adapt the displacement of the engines to the power target. Savings – particularly in view of the cost of production – can also be achieved by reducing the number of cylinders at constant displacement, as FEV studies have shown.

Commercial vehicle engines must comply with harsh exhaust emission limits even under full load. In addition, a comparatively low nominal speed and high expectations for the durability of the maximum specific power represent limitations. While automotive engines already reach values of more than 70 kW per liter, heavy commercial vehicle engines have not yet gone beyond a maximum of about 35 kW per liter.

Decreased number of cylinders, identical power, equivalent displacement

“A trend is developing with regard to rightsizing in commercial vehicle applications,” explains Dr. Peter Heuser, Group Vice President, Commercial Vehicles, Industrial and Heavy-Duty Engines. “In recent years, all of the major commercial vehicle manufacturers engines have introduced 10L class engines into the market and, thus, the gap between the conventional 7L medium-duty and the 12L heavy-duty engine has been closed.”

>>IF YOU REPLACE A 6-CYLINDER ENGINE WITH A 4-CYLINDER THAT HAS 1.5-TIMES LARGER INDIVIDUAL CYLINDER DISPLACEMENT, COST OF PRODUCTION, INSTALLATION SPACE AND WEIGHT CAN BE REDUCED WHILE, EFFICIENCY IS IMPROVED

Investigations by FEV indicate that the production cost of an engine with a predetermined performance can be effectively reduced by reducing the number of cylinders and then increasing the displacement of the remaining cylinders, accordingly.

This can be achieved by replacing a six cylinder engine with a four cylinder engine that has a 1.5 times larger single-cylinder displacement. In addition to the reduced cost, the required installation space and the weight can be reduced while, at the same time, achieving higher efficiency. The deterioration of NVH performance due to the greater ignition interval and the free second order inertia forces can be almost fully offset with a dual-mass flywheel and mass balancing shafts. These additional components were included in the review in terms of cost, size, weight and fuel consumption.

Cost savings through modularity

Four cylinder engines with a cylinder displacement of about two liters, which is standard for heavy-duty-engines open up more than just the potential cost savings of reducing the number of cylinders. The fact that a larger power range can be covered with a common cylinder diameter allows modularity and parts commonality that leads to improvements across the entire engine family.

Rightsizing - Comparison of a 4-cylinder engine with a 6-cylinder

Comparison of a 4-cylinder engine with a 6-cylinder with equal displacement

Rightsizing - MD and HD Engine Family

MD and HD Engine Family

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