The development of the Mercedes-Benz A-Class: The petrol engines - Powerful, responsive and super-clean

Mercedes-Benz has 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 great responsiveness and exemplary power delivery with outstanding efficiency and the best emission figures in this class. The world-first CAMTRONIC, an innovative valve lift adjustment feature, makes a major contribution to this. There is a choice of 1.6 or 2-litre variants of the new four-cylinder engine family for the A-Class, covering a power range from 90 kW (122 hp) and 200 Nm to 155 kW (211 hp) and 350 Nm.

Thanks to BlueDIRECT and highly precise piezo-injection, the new engines are already expected to meet the Euro-6 emission standard for petrol engines coming into force from 2015. Even the much more stringent diesel particulate limit in the Euro-6 standard is already bettered with no additional exhaust aftertreatment. This was a major motivation for introducing BlueDIRECT into this segment as well. This means that the completely newly developed generation of four-cylinder petrol engines is ideally prepared for the future.

Extremely versatile

The basis for all three petrol 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 is extremely versatile, and will also be gradually introduced into larger model series. In this way it also contributes to a sustained reduction in production costs. 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 BlueDIRECT family has grown: three new petrol engines

The M270 engine is available in three power classes in the new A-Class. As the A 180 BlueEFFICIENCY with a displacement of 1595 cc it develops 90 kW (122 hp) and a maximum torque of 200 newton metres over a wide engine speed range from 1250 to 4000 rpm. With the basic engine and the 6-speed manual or seven-speed dual clutch transmission (figures in brackets) the A-Class accelerates from zero to 100 km/h in 9.2 seconds (9.1 seconds), and has a top speed of up to 202 km/h (202 km/h). The fuel consumption and CO2 emission figures of 5.5 l/100 km (5.4 l/100 km, both NEDC combined) resp. 128 g/km (127 g/km) are so low that they possibly set a new record.

The A 200 BlueEFFICIENCY with the same displacement of 1595 cc develops 115 kW (156 hp) and a maximum torque of 250 Nm, which is likewise available from 1250 to 4000 rpm. It sprints from zero to 100 km/h in 8.4 seconds (8.3 seconds) and has a top speed of 224 km/h (224 km/h), but is happy with an average 5.5 l/100 km of fuel (5.4 l/100 km), corresponding to CO2 emissions of 129 g/km (127 g/km).

The new, top engine variant with 1991 cc is available as the A 250 BlueEFFICIENCY or A 250 Sport. In both cases the engine develops 155 kW (211 hp) and a maximum torque of 350 Nm from 1200 to 4000 rpm. Performance is at sports car level, with acceleration from zero to 100 km/h in 6.6 seconds and a top speed of 240 km/h. Nonetheless it has the best fuel consumption in this class at 6.1 l/100 km, and CO2 emissions of 143 g/km (A 250 BlueEFFICIENCY).

All engine variants are designed for customer-friendly operation with E10 premium petrol (95 RON).

Overview of technical data for the A-Class with petrol engine
Engine technology from the S-Class for the A-Class

The technology package in the new four-cylinder petrol engines includes a number of new developments which were introduced in 2010 with the ultra-modern BlueDIRECT V6 and V8 engines for the Mercedes-Benz S-Class, and are now available in the compact class for the first time.

The combustion process is based on third-generation Mercedes-Benz direct injection with highly precise, multiple piezo injection technology. In 2006 Mercedes-Benz was the first car manufacturer to introduce direct injection with spray-guided combustion into series production.

Piezo injectors for very low particulate emissions below the Euro-6 limits

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 percent. The overall result is that all emission figures including particulates are now already below the limits set by the Euro-6 emission standard, which only comes into force from 2015.

Compared with conventional multi-hole solenoid valves, piezo injectors have numerous advantages in petrol engines. The fuel vaporises 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 millimetre. The central component of a piezo-electric injector is the piezo-stack, which directly controls the metering needle. With a response time of just 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.

Multi-spark ignition for optimal efficiency

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 percent are possible alone by the use of piezo-electric injection technology in combination with multi-spark ignition, depending on the driving cycle.

CAMTRONIC: up to ten percent lower fuel consumption

A trailblazing innovation known as CAMTRONIC is celebrating its debut in the 1.6-litre 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 realised. 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. This world first was developed completely in-house by the Mercedes-Benz Technology Center in Stuttgart and the Daimler engine plant in Berlin.

No turbo-lag thanks to scavenging

Above all, Mercedes-Benz engineers were able to obtain a high output from a small displacement by the use of a turbocharger. This 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 degrees Celsius make optimal use of the exhaust gas energy, producing a high output and outstanding 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 so-called 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" or delay when moving off. 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.

Torque levels like a diesel

As a result the new 1.6-litre four-cylinder in the A-Class already delivers its maximum torque at 1250 rpm, maintaining it to 4000 rpm. The 2.0-litre even does a little better: its maximum torque of 350 Nm is already available from 1200 rpm, which means that for low-end torque it leads the field for four-cylinder engines.

In combination with the new 7G-DCT transmission, whose control unit
communicates with the engine management system, the new four-cylinder responds to the accelerator very readily. At cruising speed a higher ratio can be chosen, which further improves fuel consumption and noise level. The turbocharged engines also maintain their exemplary fuel consumption under high loads, as the outstanding cooling system only makes a richer mixture necessary at speeds above 200 km/h. A two-piece water jacket with optimised cross-flow cooling and fine, only three-millimetre wide cooling ducts between the spark plugs and injectors bring the coolant to the right places.

Thermal management shortens the warm-up phase

A new thermal management system has also been developed: in cold state, a switchable water pump with flow-optimised 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-optimised ball valve. In the interest of high efficiency, the volumetric flow of the oil pump is also controlled as in the V engines.

Controlled oil pump with two pressure stages

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. Thanks 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.

New coolant ducting and 3-phase thermal management

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 degrees Celsius has been reached in normal operation (87 degrees Celsius 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. This means that 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.

First use of Lanchester balancer shifts with antifriction bearings in a petrol engine

The secondary inertia forces inherently occurring in a four-cylinder in-line engine are compensated by two Lanchester balancer shafts in the bottom of the engine block. Mercedes-Benz is the first automobile manufacturer to use antifriction bearings to balance masses: imbalances are compensated by cylinder roller bearings, with axial forces from the gearing taken up by a ball bearing. This arrangement not only improves driving comfort, but also helps to lower fuel consumption thanks to considerably less friction. In view of the more favourable connecting rod configuration, the 1.6-litre version requires no Lanchester dynamic balancing.

Minimised friction

Particular attention was also paid to reduced friction. This was primarily achieved by a reduction in flow through the oil and water pumps, low-friction pistons, piston rings and cylinder walls, plus the new thermal management system and chain drive.

The engine and transmission are mounted at four points by an engine mounting, a transmission mounting and two pendulum supports. These are specially configured for the needs of the high-torque engine variants with a view to maximum noise comfort. The hydraulic damping integrated into the transmission mounting also contributes decisively to the excellent driving comfort.

Under the microscope: CAMTRONIC valve lift adjustment. Using the camshaft to accelerate

Mercedes-Benz is the first automobile manufacturer to equip the 1.6-litre version of the M 270 engine with 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 are mounted on the carrier shaft.
Mercedes-Benz engineers refer to these as "cam-pieces", of which the first controls the intake valves of cylinders 1 and 2, and the second those of cylinders 3 and 4. The cams themselves are masterpieces of the caster's art: they take 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.

"Accelerating" with the camshaft

Load control with the smaller valve lift is realised 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. In popular terms one might also say that the new Mercedes-Benz A-Class also accelerates with its camshaft.

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 synchronisation 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 excels by virtue of its significantly smaller dimensions. The installation space along the engine's longitudinal and vertical axes can therefore be made very compact.

Under the microscope: engine test benches - Trial by ordeal

Before the new engines were allowed onto the roads for practical endurance trials, they had already absolved a series of tortures on the engine test benches of the test facility in Untertürkheim. 24 of the very latest engine test benches are installed on each floor of this imposing three-storey building. These 72 test benches are in operation by day and night, on 365 days of the year.
The test-benches can be used to simulate a wide variety of road and load cycle situations to reflect every conceivable operating profiles, e.g. hot and cold-starting, stop-and-go traffic and long-distance operation under a wide variety of conditions. Even steep mountain gradients can be simulated in the laboratory: a pivoting test rig is e.g. used to tilt the engine by up to 40 degrees to examine the effects on the oil circuit.

At various development stages the engines are subjected to accelerated stress tests. "This simulates stresses that no customer can achieve," says Thomas Uhr, who is responsible for the workshops at the Mercedes-Benz development centre and for powertrain testing. The test-bench programmes have a duration of 500 to 2400 hours, with large proportions under full load and partial load. Thermal characteristics and component durability are tested under full load conditions, however particularly low loads can also be a real torture for an engine in the form of e.g. engine oil sludging. Other tests include heavy stresses such as cooling the coolant from 110° C to approx. 25° C within a max. of 60 seconds, and running the engine up to its rated speed under full load when the coolant is cold. Thomas Uhr: "We test our engines more intensively than any other manufacturer."
The new OM 607 diesel engine absolved more than 25,000 hours on the test-benches, for example, before it received approval for use in the A-Class.

The energy generated by the test engines is recovered as fully as possible. The engine testing facility in Untertürkheim is certificated as a combined heating and power plant: only active equipment is used to monitor and measure engine power. This power is not used to propel a vehicle as in normal use, but rather converted into electrical energy and fed into the plant's power network.
In addition to long-term durability, fuel consumption, emissions and driveability in conjunction with the transmission are the major development goals. This requires enormously painstaking dynamometer tests followed by practical trials on the roads.

Credits: Daimler AG

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