Engineers from Mercedes first introduced gasoline direct injection nearly 70 years ago with their supercharged V12 DB601 aero engine. The technology has been revisited, although for a somewhat more serene application.
As reported in AE March, Mercedes claimed a world first by unveiling a petrol engine featuring piezoelectric direct injection with spray-guided combustion. The new powerplant appears first in the CLS class coupé. The CLS 350 CGi will be available from the third quarter 2006, offering more power and torque than the previous model but with reduced fuel consumption and emissions.
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CLS 350
gets new DI V6
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The 3.5 litre 90° V6 develops 215kW/365Nm and hauls the car to 62mph in as little as 6.5 seconds before reaching an electronically limited top speed of 155mph. Best fuel consumption on the combined cycle is 9.1 litres/100km (31mpg), a reduction of more than 10 per cent over the current CLS 350.
Second generation gasoline direct injection (DI) operating under stratified (lean) conditions offers greater fuel economy and efficiency than when operating with a homogeneous mixture. Until now, the effectiveness of stratified charges has been limited by control of the combustion process – lean only in low rpm – and by exhaust emissions with excessive nitrogen oxides (NOx). Mercedes engineered the CGi powertrain to address these issues.
Lean mixtures, above the ideal, stoichiometric air:fuel ratio of 14.7:1 (l=1), enable more total combustion of the fuel because of the presence of excess air.
Stratification is achieved by injecting the fuel, at high pressure, late in the compression stroke and as close to the spark plug as possible.
In so doing, the fuel is atomised as completely as possible but is still surrounded by the remaining charge air, hence “stratified”. The unburnt air also reduces temperatures in the combustion chamber, which has two benefits: inhibiting formation of NOx and reducing the tendency for pre-ignition. Designed for 95 octane fuel, a high compression ratio of 12.2:1 is used (10.7:1 for original CLS 350), raising engine efficiency still further.
Wall guided delivery
First generation DI relies on wall-guided combustion: the injected stream of petrol deflects from piston crown to form an air/fuel mixture prior to ignition. In so doing, a small quantity of fuel will always condense on the surface, leading to incomplete combustion, which is detrimental to efficiency and hydrocarbon (CO) emissions.
New piezo injectors developed by Bosch, relocated to the top of the combustion chamber, produce a very narrow annular gap through which fuel is released at 200bar. A very precise, hollow cone shaped spray is formed, directed straight downwards and stable under all charge conditions.
When operating in stratified mode, there are two injections, 50µs apart, before TDC on compression, with a post-injection just after TDC under part load conditions.
Under stratified conditions, the engine can operate as lean as l=8 and extends stratified operation into higher rpm and load ranges, before switching to homogenous combustion. This is a major contributing factor in the fuel consumption reduction over the port injected engine it replaces.
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Fuel system uses Piezo fuel injectors |
Piezo injector technology has a proven record in direct injection diesel engines, first appearing in 2000. Piezo actuation improves on solenoid actuation through significantly faster operation, with switching speed an order of magnitude quicker, and unlike solenoids, opening the needle directly, and through multiple injections per cycle of greater repeatability.
Emissions control is applied from two aspects: in-cylinder, and after-treatment. The combustion process itself greatly reduces CO and HC, and exhaust gas recirculation (EGR) via two large bore EGR valves helps to reduce NOx at source.
The exhaust system is fitted with four catalytic converters: two three-way catalysts and two NOx storage catalysts, each with two sensors.
Lean operation (lambda>1) creates higher levels of NOx because excess oxygen in the exhaust stream prevents the three-way catalyst from converting these compounds into nitrogen gas: under stoichiometric conditions (lambda=1), CO and HC emissions are oxidised into CO2 and H2O by simultaneously reducing NOx to N2. This cannot occur at lambda>1 because the oxygen in the exhaust gas is used in preference, allowing the NOx to pass through untreated, hence the need for NOx absorption catalysts.
NOx is absorbed into the barium coating of the catalyst substrate during lean operation, and converted into barium nitrates. To regenerate the catalyst and clear it of stored NOx, the engine has to switch to rich running conditions (lambda<1). This occurs automatically under full load or is momentarily effected by the engine ECU, triggered by a signal from the NOx sensors.
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Emissions aftertreatment strategy is complex |
One major issue with NOx storage catalysts is an acute sensitivity to the sulphur in fuel: if present in high enough concentrations, it forms sulphate compounds on the catalyst substrate where they accumulate, reducing its effectiveness and eventually destroying it.
For this reason, Mercedes will not sell the CGi variant in markets that do not have low sulphur fuel (less than 10ppm), which currently includes Russia and China.
Variable valve timing
Other technical features of the engine include fully variable valve timing for both inlet and exhaust, variable length inlet tracts to bolster low range torque and all-aluminium construction with a low friction Al-Si coating of the cylinder bores.
Located between the two banks of cylinders is a contra-rotating balancer shaft to offset the vibrations caused by the first and second order free moments inherent to the V6 configuration. The block is shared with the port injected engine and both are built in the same plant.
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New V6 restricted to CLS at present |
This new engine uses an impressive array of technologies to reduce fuel consumption and emissions while improving output and performance. However, it is considerably more complex and therefore expensive, than the existing V6, which is used in the new E-Class saloon.
At present, the CGi V6 is used only in the CLS coupé. Just by doubling the number of catalysts, sensors and requisite wiring looms used in the exhaust system, considerable cost has been introduced, and yet these components are used as a direct consequence of using a stratified-charge combustion process; the one necessitates the other.
Looking forward to Euro V and beyond, emissions control will become paramount and so direct injection will be more widely used; Bosch believe annual production of DI gasoline engines will exceed three million units by 2010.
At the same time, if gasoline is to close the fuel economy gap on diesel and hybrids, it is clear that significant advances are required. As the principal method of carburation, port fuel injection will be replaced, in the same way that it once replaced the carburettor.
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