|The new CAMTRONIC variable valve lift adjustment system debuts in the 1.6-liter gasoline engine. Click to enlarge.|
The new Mercedes-Benz A-Class, due to begin sales in Europe in September 2012, features a wide choice of gasoline and diesel engines with up to 26% lower fuel consumption compared to comparable preceding models, accompanied by a considerable power increase.
The A 180 CDI will be the first Mercedes-Benz to emit only 98 g of CO2 per kilometer; the A 220 CDI is the first Mercedes to meet the Euro-6 emission standard which comes into force in 2015. All engines of the new A-Class feature the ECO start/stop function as standard. The engines can be combined with the new six-speed manual transmission or optionally with the 7G-DCT dual clutch automatic transmission.
Diesels. The new basic engine in the OM 607 series develops 80 kW (109 hp), delivers 260 N·m (192 lb-ft) to the crankshaft and with a manual transmission consumes 3.8 liters per 100 km (62 mpg US), corresponding to 98 g of CO2/km. This is a 22% improvement over the 60 kW (82 hp) preceding model, the A 160 CDI, which consumed 4.9 liters (48 mpg US). The new top diesel, the A 220 CDI, is 25% better than its predecessor: it develops an output of 125 kW (170 hp) and 350 N·m (258 lb-ft) of torque, and in combination with the 7G-DCT automatic dual clutch transmission it consumes 4.3 liters/100 km (55 mpg US) (provisional figure). The figures for the preceding A 200 CDI were 103 kW (140 hp), 5.7 liters (41 mpg US), 149 g of CO2.
For transverse installation in the A-Class, the the belt drive for the ancillary units, the installed position of the turbocharger and the air ducting of the engines have been modified.
The 220 CDI is only available in combination with the 7G-DCT dual clutch transmission. The 125 kW (170 hp) top diesel is equipped with a weight-optimized crankshaft with individual bearing covers bolted from below and four counterweights, enabling it to weigh around six kilograms less than a longitudinally installed OM 651 of the same displacement. The single-stage turbocharger has larger dimensions than that in the 80 and 100 kW variants. The A 200 CDI has multiple exhaust gas recirculation to reduce nitrogen oxide emissions. It already meets the Euro-6 emission standard coming into force from 2015.
With 112 g of CO2 per kilometer (provisional figure) the A 220 CDI sets new standards in the segment. With a displacement of 2.2 liters the engine is comparatively large, and therefore already agile at low rpm—i.e., downspeeding (the combination of a large displacement and low engine speeds). As a result it has been possible to make the ECONOMY mode of the 7G-DCT transmission more economical and comfortable. If the driver selects “S”, gearshifts are performed much faster and the ratio spread uses the rpm reserves of the engine for dynamic performance.
The 1.8-liter engine variant is used in the A 180 CDI with the 7G-DCT dual clutch transmission and the A 200 CDI. The displacement was reduced by shortening the stroke (83 mm instead of 99 mm). The significantly longer connecting rods ensure lower transverse friction, and the two Lanchester balancer shafts are also of low-friction design. The single-stage turbocharger was optimized for efficiency and features adjustable vanes.
With a distance between cylinders of 94 millimeters and cylindrical gears driving the camshafts, transverse installation and the necessary length restriction were part of the design specification from the start. To realize the start/stop function, the belt drive is decoupled from the crankshaft in all three engines.
Other features common to all the diesels include:
Common rail technology with a rail pressure increased to 1800 bar. The maximum ignition pressure of 180 bar also contributes to the high power output and a muscular torque curve.
The oil injection nozzles and the water pump are activated only when required, in order to save energy and fuel. The controlled oil pump additionally reduces oil flow and thus fuel consumption.
The engine block is made of cast iron, the cylinder head of aluminium.
A two-piece water jacket in the cylinder head provides for optimum cooling in the area of the combustion chamber plate. This enables an ignition pressure of 200 bar and a high specific power output.
The cast iron barrels have undergone considerably finer honing than on the predecessor, also contributing to the reduction in fuel consumption.
To compensate for the second-order forces which are inherent to four-cylinder in-line engines there are two Lanchester balancer shafts at the bottom of the engine block running in low-friction roller bearings rather than conventional plain bearings.
The two-mass flywheel has been specifically designed for high engine torque at low engine speeds in order to isolate the crankshaft’s vibration stimuli, thereby contributing to smooth running.
|Multiple EGR. Click to enlarge.|
Multiple EGR. To reduce NOx emissions even further, the OM 651 engine of the A 220 CDI is equipped with multiple exhaust gas recirculation (EGR). In addition to high-pressure EGR, where hot exhaust gases are taken from the manifold and reintroduced on the fresh air side, downstream of the intercooler, exhaust gases are diverted at a lower pressure level. This low-pressure EGR diverts the filtered exhaust gases downstream of the diesel particulate filter, cools them and uses a valve to return them to the fresh air flow upstream of the turbocharger.
NOx generation is primarily influenced by the oxygen concentration in the combustion chamber (= proportion of exhaust gases). A further increase in EGR rates using classic high-pressure EGR has the inherent disadvantage of charging losses and further throttling to achieve the necessary scavenging gradient. This leads to disadvantages with respect to particulate emissions and fuel consumption. Low-pressure EGR solves these problems, as it does not reduce the drive energy of the turbocharger while at the same time considerably reducing the throttling requirement of a high-pressure EGR system.
Gasoline. With 115 kW (156 hp) and 250 N·m (184 lb-ft) of torque, the new engine delivers superior performance with consumption of 5.5 liters/100 km (43 mpg US) (129g CO2/km), which is 26% less than its predecessor (100 kW, 185 N·m, 7.4 l/100 km, 174 g CO2). Even the new top model with 7G-DCT, 155 kW (211 hp) and 350 N·m is more efficient with a consumption of 6.1 liters (39 mpg US) and CO2 emissions of 143 g.
For these engines, Mercedes-Benz systematically transferred the BlueDIRECT technology of the V6 and V8 engines in the Mercedes-Benz luxury class. The BlueDIRECT four-cylinder petrol engines for the new A-Class combine responsiveness and power delivery with efficiency and the best emission figures in this class. The new CAMTRONIC valve lift adjustment feature, makes a major contribution to this.
CAMTRONIC makes its debut in the 1.6-liter engine. For the first time in a turbocharged direct-injection engine, a load management system with an earlier intake cut-off and intake valve lift adjustment has been realized. This reduces the throttle losses under partial load, lowering fuel consumption. In the New European Driving Cycle (NEDC), fuel consumption is reduced by three to four percent compared to the M 270 without CAMTRONIC. In day-to-day driving, which typically has a high proportion of partial load operation, the potential saving is even greater, with fuel savings of up to ten percent in certain operating ranges. CAMTRONIC was developed completely in-house by the Mercedes-Benz Technology Center in Stuttgart and the Daimler engine plant in Berlin.
There is a choice of 1.6 or 2-liter variants of the new four-cylinder engine family for the A-Class, covering the power range from 90 kW (122 hp) and 200 N·m to 155 kW (211 hp) and 350 N·m.
With BlueDIRECT and precise piezo-injection, the new engines are expected to meet the Euro-6 emission standard for gasoline engines coming into force from 2015. Even the much more stringent diesel particulate limit in the Euro-6 standard is already bettered.
The basis for all three gasoline engine variants in the new A-Class is the all-aluminium M 270 engine with two chain-driven overhead camshafts and four-valve technology. This power unit will also be gradually introduced into larger model series. The four-cylinder can be installed transversely (M 270) or longitudinally (M 274), and combined with front, rear or 4MATIC all-wheel drive, and also with a manual, automatic torque converter or dual clutch transmission.
The technology package in the new four-cylinder gasoline engines includes a number of new developments which were introduced in 2010 with the BlueDIRECT V6 and V8 engines for the Mercedes-Benz S-Class, and are now available in the compact class.
The combustion process is based on third-generation Mercedes-Benz direct injection with multiple piezo injection technology. The newly developed piezo injectors allow up to five injections per power stroke.
In the warm-up phase this enables particulate emissions to be reduced by more than 90%. The overall result is that all emission figures including particulates are now already below the limits set by the Euro-6 emission standard.
Compared with conventional multi-hole solenoid valves, piezo injectors have numerous advantages in gasoline engines, Daimler says. The fuel vaporizes up to four times as fast, the jet of fuel penetrates less deeply into the combustion chamber and the injectors are able to deliver minute quantities of fuel extremely precisely. All this prevents fuel from being deposited on the combustion chamber walls, resulting in significantly reduced particulate emissions. Moreover, multiple injections allow operating strategies for maximum fuel efficiency while improving cold-start characteristics.
The crystalline structure of the piezo-ceramic changes in microseconds under an electric voltage, and with a precision of just a few thousandths of a millimeter. The central component of a piezo-electric injector is the piezo-stack, which directly controls the metering needle. With a response time of 0.1 milliseconds, the fuel injection can be very sensitively and precisely adjusted to the current load and engine speed, with a beneficial effect on emissions, fuel consumption and combustion noise.
The third-generation direct injection system also features “rapid multi-spark ignition” (MSI). Following the first spark discharge and a brief combustion period, the coil is recharged rapidly and a further spark is discharged. The MSI system enables up to four sparks to be discharged in rapid succession within one millisecond, creating a plasma with a larger spatial expansion than conventional ignition.
The rapid multi-spark ignition can be actuated to vary both the timing of the sparks and the combustion period to suit the relevant operating point. This provides scope for the best possible centre of combustion and improved residual gas compatibility. This in turn reduces fuel consumption. Fuel savings of up to 4% are possible alone by the use of piezo-electric injection technology in combination with multi-spark ignition, depending on the driving cycle.
The turbocharger forces the intake air into the combustion chambers at a pressure of up to 1.9 bar, with the turbine vanes rotating at up to 230,000 rpm. The charger has been designed to deliver high torque even at low engine speeds. A newly developed manifold turbocharger module is integrated and positioned in front of the engine for the best possible cooling. Separate exhaust ducting from the cylinders to the turbocharger and the high exhaust temperature of up to 1050 °C make optimal use of the exhaust gas energy, producing a high output and responsiveness.
By using a combination of direct injection and variable adjustment of the intake and exhaust camshafts, the developers were also able to exploit the advantages of scavenging: partly overlapping the opening times of the intake and exhaust valves causes some of the cold intake air to flush the hot exhaust gas from the cylinder into the exhaust manifold, which considerably improves charging compared to conventional operation.
Especially at low engine speeds, and thanks to the increased mass flow in the exhaust tract, the turbocharger also responds much more rapidly—this completely avoids any turbo-lag. The direct injection system ensures that the fresh gas is not yet mixed with fuel when it enters the cylinder, as would be the case in engines with manifold injection. There are therefore no undesirable scavenging losses—i.e. unburned fuel flushed into the exhaust manifold.
A new thermal management system has also been developed: in cold state, a switchable water pump with flow-optimized ball valve ensures that no coolant flows through the engine, providing for swift heating-up of the combustion chambers after starting up the engine. The thermostat is electronically controlled and the coolant temperatures are adjusted according to driving style and ambient conditions. The thermostat itself is also a flow-optimized ball valve. In the interest of high efficiency, the volumetric flow of the oil pump is also controlled as in the V engines.
The variable vane-type oil pump operates with two pressure stages, depending on the characteristic map. At low engine speeds and loads the pump runs at a low pressure of two bar. At this time the oil-spray nozzles for piston cooling are switched off. The high-pressure stage is activated at the upper load and engine speed levels. Due to this control concept, depending on engine load and engine speed the lubrication and cooling points of the engine can be supplied with significantly lower drive energy than would be possible with an uncontrolled pump.
The coolant ducting in the cylinder head is also completely new. The water mantle is of two-piece construction to improve flow. This leads to specific increases in flow speeds and heat dissipation at certain points, accompanied by a rigorous reduction in pressure losses throughout the coolant circuit. This has made it possible to reduce the power output of the water pump despite an increased engine output.
As it warms up, the flow of coolant is regulated by a 3-phase thermal management system so that it rapidly reaches normal operating temperature. Initially the coolant remains at rest in the engine. It then circulates in the engine circuit, but without the radiator. When a temperature of 105 °C has been reached in normal operation (87 °C under high load), the vehicle’s radiator is included in the circuit.
ECO start/stop function with direct-start. The start/stop system included as standard in all models operates with starter-supported direct-start. When the engine is switched off, the attitude of the crankshaft is registered by a new crankshaft sensor so that the engine control unit knows the positions of the individual cylinders. On restarting, it can then select the cylinder that is in the most suitable position for first ignition. After the starter has briefly turned over the engine, reliable injection, ignition and combustion is immediately possible in the ideally positioned cylinder.
CAMTRONIC intake valve lift adjustment. The system operates mechanically, but is served by an electronically controlled actuator. The intake camshaft is made up of several components: two hollow-drilled sub-shafts of equal size (cam-pieces) are mounted on the carrier shaft. The first cam-piece controls the intake valves of cylinders 1 and 2, and the second those of cylinders 3 and 4.
The cams have the form of a double-cam with two curved surfaces. The surface operating the valves via roller-type rocker arms is only half as wide as on a conventional cam, therefore the space requirement is the same. When the steeper half of the cam is active, the valve lift is increased and the valves remain open for longer. Switching to the flatter half of the cam shortens the valve lift and the valves close sooner.
Load control with the smaller valve lift is realized using various components. At very low engine torque the load control is conventional, using the position of the throttle flap, at medium torque levels using the position of the intake camshaft and at high torque levels using the charging level of the turbocharger.
As the torque increases the valve lift is switched to the larger level, load control once again being conventional via the throttle flap or, in the charged operating range, via the charging level of the turbocharger.
Mercedes-Benz development engineers took numerous measures to ensure the most efficient combustion even with the smaller valve lift. Owing to the smaller valve lift and early intake valve closure, the turbulence in the combustion chamber is reduced at the spark plug. This turbulence decisively influences the combustion speed and full combustion of the fuel/air mixture. To compensate this apparent disadvantage, the turbulence is increased in the lower partial load range by using a multiple injection strategy with injection ignition, while multi-spark ignition ensures reliable combustion.
The switchover from the smaller to the larger valve lift goes unnoticed by the driver. As cylinders 1 and 2 as well as 3 and 4 are coupled in pairs with one cam-piece each, it is possible to adjust the valve lift of all four cylinders within one camshaft revolution using just one double actuator. A correspondingly large effort was required to develop the synchronization for the switching process and ensure the long-term durability of the components.
The variable, hydraulic vane-type camshaft adjusters on the intake and exhaust sides have a wide adjustment range of 40 degrees with reference to the crankshaft. This new development features significantly smaller dimensions.