|The basic concept of GM’s variable swirl mechanism. Click to enlarge.|
The 2014 Chevrolet Cruze Clean Turbo Diesel’s (earlier post) 2.0L engine uses a variable-swirl intake mechanism to enhance performance while reducing emissions. The variable-swirl intake manifold design optimizes air charge mixture motion in the cylinders for a more efficient combustion.
In Cruze’s variable-swirl intake manifold, each cylinder has two separate intake ports, with one of them controlled by a valve. During variable swirl, each throttle valve varies how much it opens to create mixture-motion of air and fuel within each cylinder. The effectiveness of any such variable mechanism is linked to the quality of its controls; GM’s engineering team spent a great deal of effort calibrating the actuator precisely to open and close the valves for optimal performance.
Use of variable swirl to enhance combustion has been studied in the industry literally for decades, with some work reaching back to the 1960s. GM was awarded a patent on one approach to an engine intake port with variable swirl vanes in 1984 (US Patent Nº 4,432,312). Optimizing swirl speed depending upon the engine operating condition is an effective means to increase thermal efficiency and reduce emissions, Toyota researchers noted in a paper in the R&D Review of Toyota CRDL in 2002.
In a study published in 2000, Dr. Paul Miles at Sandia National Laboratories found that:
Swirl ratio is found to exert a measurable influence on most of the combustion process, from ignition to late-cycle oxidation. Ignition delay decreases with increasing Rs, as do the magnitudes of the initial premixed burn, the peak rates of heat release, and the maximum rates of pressure rise.
A fixed geometry approach to providing fuel-air swirl is less than ideal because, in part, the amount of swirl delivered for part load is less than desired; on the other hand, the fixed geometry devices can restrict inlet flow under full load conditions. Thus, a range of approaches—including specially chamfered variable lift valves—have been developed to provide variable mechanisms to provide high swirl at lower loads and low restriction at full loads.
All combustion engines have swirl, but having variable swirl is a real difference maker for Cruze’s diesel engine. Ultimately, it means more driving satisfaction with less environmental impact.—Mike Siegrist, 2.0L turbo diesel assistant chief engineer
Cruze’s diesel engine, mated to an automatic transmission, provides greater fuel economy than a comparably sized gasoline engine through greater thermal efficiency, a higher compression ratio and an unthrottled combustion process. Cruze’s diesel offers a segment-leading EPA-estimated 46 mpg highway (5.1 l/100 km) and range of more than 700 miles per tank.
Cruze Clean Turbo Diesel’s emissions will meet US Environmental Protection Agency’s (EPA) Tier 2 Bin 5 emissions standards.
Miles, P. (2000) The Influence of Swirl on HSDI Diesel Combustion at Moderate Speed and Load, (SAE Technical Paper 2000-01-1829) doi: 10.4271/2000-01-1829