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GM fields variable valve lift -- Finally!

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DETROIT – Drivers of the new 2014 Chevrolet Impala will enjoy sweet sound and sweeter savings at the pump thanks to precision noise reduction and a new advanced valvetrain technology on the Ecotec 2.5L four-cylinder engine, arriving in dealerships this month.

Chevrolet expects the 2.5L model to be a popular choice among Impala buyers. More than two-thirds of Chevrolet cars sold in the first quarter of 2013 had a four-cylinder engine.

“Impala customers have three engines to choose from, appealing to a wide array of needs,” said Chris Perry vice president, Chevrolet Marketing. “But the 2.5L model in particular offers both improved fuel economy as well as an accessible starting price of $27,535.”

Chevrolet expects its new flagship sedan to be more fuel efficient, due in part to the 2.5L engine’s Intake Valve Lift Control technology.

In older engines, valves operate strictly based on mechanical camshaft rotation, opening and closing the same way all the time. Chevy’s new system allows the valves to open and close by varied amounts and at different times depending on power demand to provide greater fuel efficiency or power.

When the technology operates in low-lift mode, the engine pumps only the air it needs to meet the driver’s demand. The system switches to high-lift mode at higher engine speeds or under heavy loads, providing the full output capability of the engine.

Impala’s 2.5L engine, which delivers SAE-certified 196 horsepower (146 kW) and 186 lb-ft of torque (252 Nm), achieves variable valve lift using an all-new rocker arm that switches between low and high lift intake cam profiles. The mechanism is actuated by an oil control valve through a dual-feed stationary hydraulic lash adjuster. It is the first of its kind for low friction roller-type finger-follower valvetrains in gasoline engines. The engine’s computer continuously selects the optimal lift profile based on conditions such as engine speed and load.

“Intake Valve Lift Control works so seamlessly drivers aren’t likely to notice it at all,” said Mike Anderson, General Motors’ global chief engineer for Ecotec engines. “What they will notice is a fuel economy improvement of up to one mile per gallon.” The EPA estimated fuel economy for the 2014 Impala with the new 2.5L engine is 21 mpg city and 31 mpg highway.

The redesigned large sedan’s 2.5L engine with direct injection is engineered to be one of the quietest and most refined in the segment. The development team reduced engine noise intensity by 40 percent by specifically targeting the 2.5L’s noise frequency signature. They pushed radiated noises into a higher frequency range well above 2,000 hertz, which is more pleasing to the ear – particularly in the high-load operating ranges where engine sound is most intense.

The refinement-enhancing changes and improvements over previous Ecotec engines ranged from the comparatively simple – such as integrating a sound-absorbing cover into the intake manifold and specifying quieter drive chains – to more fundamental architecture items, such as relocating the balance shafts from the cylinder block to a cassette within the oil pan.

Impala’s passengers get a quieter driving experience due in part to active noise cancelling technology and a more refined sound as the engine revs to its 7,000-rpm peak – a sound that GM Noise and Vibration Engineer Tom Slopsema likens to the precision purr of a sewing machine.

“We focused on reducing the overall engine noise level and placing the remaining noise in a higher frequency range,” Slopsema said. “No fastener, cover or internal engine part was left unexamined in our quest to make this engine one of the quietest in the industry.”

Impala offers three fuel-efficient powertrains, including the 3.6L V-6, the new Ecotec 2.5L four-cylinder and the Ecotec 2.4L with eAssist.

Founded in 1911 in Detroit, Chevrolet is now one of the world’s largest car brands, doing business in more than 140 countries and selling more than 4.5 million cars and trucks a year. Chevrolet provides customers with fuel-efficient vehicles that feature spirited performance, expressive design, and high quality. More information on Chevrolet models can be found at www.chevrolet.com.

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The silly thing really is that, as a fuel economy device, they are not employing it effectively and the effects on fuel economy and/or performance is marginal at best.

What they should have done is create a virtually variable effective displacement / effective compression engine.

Bore x Stroke: 88 x 101 mm

Static Displacement: 2457 cc

Static Compression: 13.5:1

Low RPM mode (lower lift, extended intake duration, pseudo-Atkinson cycle)

Effective Displacement: 1966 cc

Effective Compression: 10.8:1

Effective output: 122 bhp @ 4200 rpm, 153 lb-ft @ 4200 rpm

Minimum Operating speed: 500 rpm (idle)

Maximum Operating speed: 4200 rpm

Performance (Otto) Mode

Effective Displacement: 2457 cc

Effective Compression: 13.5:1

Effective output: 250 bhp @ 7000 rpm, 200 lb-ft @ 5200 rpm

Minimum Operating speed: 2700 rpm

Maximum Operating speed: 7200 rpm (fuel cut)

The basic idea is this... By closing the intake valves really late -- 20% into the compression stroke's piston travel -- you essentially eliminated 20% of the effective displacement by kicking about a fifth of the air back out the cylinders. You also reduce the effective compression by about 20% because you are squishing 20% less air into the same combustion chamber volume. Finally, because the power (expansion) stroke remains the same you have an asymmetrical compression/expansion stroke which significantly improves energy extraction from the fuel burned. This method of operation is good for fuel economy. Most hybrids use this cycle for maximum fuel efficiency from their ICE at the expense of specific output.

But, because we have cam switching system here, there is a greater implication. Notice that a 13.5:1 compression ratio and 2.5L displacement was what we started with, and this was turned into 2.0L and 10.8:1 compression due to the Atkinson Cam? If we stop using an pseudo-Atkinson cam grind we are back at a sky high 13.5:1 compression and 2.5L displacement! The problem of course is that 13.5:1 compression will probably ping and knock even on 91 octane premium. That is why it is not used on current 2.5L non-iVLC engines. But, knocking really happens not at high rpms and at high acceleration situations, but at low rpm, high load situations. The reason being that knocking -- the ignition of the charge prior to spark ignition -- is really a function of temperature and time. Compress a fuel-air mixture to a certain degree, expose it to a certain temperature and AFTER a certain amount of time it goes "boom" without the need for a spark plug to light it. It is when the engine is turning slowly and carrying a heavy load (wide throttle opening letting lots of air and fuel in) at it is most likely to knock, because the mixture is in the cylinders the longest before the spark fires. At high rpms, there is less in cylinder time between when the fuel and air is in the cylinder and when the piston approaches top dead center and the spark plug fires. With the ability to drop effective compression at low rpms, we also have the ability to run ridiculous compression ratios because we are only going to run the Atkinson cam at low engine speeds.

The exact switchover depends on not just rpm but also throttle opening and rate of acceleration. But generally speaking it should occur somewhere between 2700 and 4200 rpm. Below 2700 we are always on the Atkinson cam which lowers displacement, increases fuel economy and most importantly drops our effective compression ratio. Above 4200 we are always on the high performance cam grind (which won't work at low speeds and will in fact ping like crazy at low speeds). About 102 hp/liter can be expected with a high performance cam with 13.5:1 compression), at the same time the engine is rendered operable and more fuel efficient than usual below 2700~4200 rpm, albiet a little low on torque and power, thanks to the Atkinson grind. When cruising on the freeway or puttering around town with a gentle foot, you really don't need all the performance you can get. A prod of the pedal brings a downshift above the 2700~4200 rpm threshold and the unleasing of a race bred 4-potter. This can be one of the best 4-cylinders ever.

Edited by dwightlooi
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BTW, if GM does the aforementioned, it actually won't be the first... Mazda's Skyactiv-G engines are exactly that. That's how they get 13:1 and 14:1 compression to work on pump gas. However, their performance cam is not particularly aggressive.

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Very interesting, would be good to have on a hybrid system and they should roll it out across the whole family of products to bump up MPG immediately.

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Well, a switching system like this can be used in the following ways...

  • Atkinson Cam <-> Race Cam -- Virtual variable displacement, variable compression, variable overlap, variable duration, variable lift (as described above)
  • Atkinson Cam <-> Regular Cam -- Virtual variable displacement, variable compression, variable overlap, variable duration, variable lift (Eg. iVTEC-E)
  • Regular Cam <-> Race Cam -- Variable overlap, variable duration, variable lift (Eg. VTEC, VVTL-i)
  • Total Cylinders Shut OFF <--> Atkinson Cam or Regular Cam -- Allows engine to free wheel without pumping losses, allows efficient, clutchless, part-time 100% electric propulsion*

* An engine so equipped will have no torque converter or clutch! Basically, it is a hybrid vehicle which can ONLY start from standstill or go below 500 rpm in 1st gear on electric power. It cannot idle the internal combustion engine. All cylinders are deactivated as the vehicle slows to a stop or accelerate from stand still. The engine is turned over relatively effortlessly by the electric motor along with transmission input, because all the valves are shut and the pistons act as springs. You always rely on 100% electric power to get going initially, the engine is started simply by reactivating all the valves and injecting fuel. Shifting once underway is done by electronic throttle modulation (for rev matching) and directly shifting the transmission without a clutch.

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