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Computer-controlled Intake valve butterflies promote intake air swirl on the GM 1.6-L turbodiesel. (Paul Weissler image)

GM's 10 steps to a quiet new pass-car diesel

When Volkswagen fell afoul of U.S. and European emissions rules, it effectively stripped the U.S. car market of small turbodiesels for passenger vehicles. While Mazda is still certifying its 2.2-L Skyactiv-D for the CX-5 (now expected on sale in spring 2018), General Motors is also eager to fill the void. Its new 1.6-L turbodiesel, a $2,200 option for the 2018 Chevrolet Cruze and Equinox, was engineered four years ago in Italy, is built in Hungary, and was calibrated in Detroit for the hitherto-abandoned market.

Fuel economy, with highway numbers particularly impressive, are diesel strong points. The Cruze diesel is EPA highway-rated at 52 mpg with the GM-Ford joint venture 9T50 9-speed automatic. The Equinox is rated at 39 mpg for 2WD, 38 mpg for 4WD with the 6-speed automatic. Equinox’ greater curb weight precludes any real-world economy advantage for the 9-speed vs. the 6-speed.


SAE-certified "flüsterdiesel"

The engine, part of GM's global MDE family, SAE-certified at 137 hp (102 kW). GM claims 90% of the 240 lb·ft (325 N·m) peak torque is available from 1500 to 3250 rpm. The engine proved extremely quiet during an Automotive Engineering road test. German autowriters first dubbed it der flüsterdiesel, or "whisper diesel." We heard no audible chatter while standing next to the car with the engine idling.

Featuring an aluminum cylinder head, block and bedplate, the engine has a 16:1 compression ratio and fixed valve timing. Performance and emissions are improved by variable intake air swirl from a shaft-mounted lineup of pivoting butterfly valves for one of each cylinder’s two intakes. The BorgWarner variable-geometry turbo machine, with peak boost of 42 psi (290 kPA), uses air-to-air intercooling which smoothes acceleration. Like most diesels, peak torque is at fast idle, providing pleasing launch.

Although there’s an acoustic foam top cover, acoustic foam-covered injectors and common rail; and clamshell manifold cover, quiet design also is engineered-in. The 1.6-L incorporates 10-step solenoid fuel injectors (each with eight holes), and rear location of timing gears and chains, where the bell housing and transaxle absorb gear/chain noise.

That level of multi-injection would be more typical of piezo injectors, admitted Mike Siegrist, GM Global Propulsion's Regional Chief Engineer. But piezo units would add an intolerable “$30 per hole” over the 1.5-L base gasoline engine, he revealed. This new generation of solenoid injectors, with twice the previous pulsation rate, he said, provides near-piezo performance with the system operating at 29,000 psi (2000 bar).

Multi-pulse solenoid strategy

Solenoids start with pre-combustion pilot injection, and follow with a varying number of rapid pre-ignition “shots” for a shortened delay before the combustion injection. These reduce NOx generation and lower noise/vibration. The combustion-ignition pulse follows, and then post combustion pulses to control exhaust gas temperature, burn off maximum amount of particulate and further lower emissions. It’s the up-to-10 pulses that produce the total results.

The EGR (exhaust gas recirculation) system incorporates liquid cooling and although a diesel normally runs unthrottled, the Ecotec 1.6 L has an electronic throttle to raise manifold pressure, to increase EGR flow as part of the engine control system, Siegrist explained.

The system incorporates post-combustion SCR (selective catalyst reduction) with urea injection for further NOx control. There is also an oxidation catalyst and particulate filter with regeneration. GM clearly approached U.S. certification of this engine aggressively to ensure low emissions.

The vehicles have an 18.5-L (4.9 gal) tank of  “diesel emission fluid” urea solution. Although manufacturers try to calibrate for a 7500-mi-plus (12,000 km) range to coincide with the oil change interval, Siegrist conceded that with the final calibrations the 1.6-L Ecotec is more likely to need a tank refill in the 5000-6000 mi (8000-9600 km) range. He explained that the urea tank monitoring system provides continuous instrument panel warnings in plenty of time for a refill.

Idle stop-start is incorporated with both automatics, and AE's road testing showed it’s barely perceptible, with restarts quick and gentle.

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