|The two-mode ML 450 HYBRID. Click to enlarge.|
Mercedes-Benz staged the world premiere of the ML 450 HYBRID at the 2009 New York International Auto Show (NYIAS) (earlier post). Fuel consumption and emissions from the ML 450 HYBRID are up to 60% less than a conventional eight-cylinder SUV with permanent all-wheel drive.
The two-mode hybrid technology in the ML 450 HYBRID is derived from the fundamental work undertaken by the Global Hybrid Cooperation, which brought together research and development work undertaken by Daimler AG, BMW Group, General Motors and Chrysler. The ML 450 HYBRID was specifically developed for the North American market.
The ML 450 HYBRID offers a system output of 250 kW (340 hp) and a combined torque of 517 Nm (381 lb-ft), with city fuel consumption of 21 mpg (11.2 L/100km) and highway consumption of 24 mpg (9.8 L/100km), along with a SULEV (Super Ultra Low Emission Vehicle) classification. The functionality of the 4MATIC permanent all-wheel drive system is also fully retained.
As a point of comparison, the 2009 Cadillac Escalade 2WD two-mode hybrid carries an EPA rating of 20 mpg city, 21 mpg highway (11.8 L/100km city, 11.2 L/.100km highway), with a Bin 5, LEV II emissions classification.
At the heart of the two-mode hybrid drive is the transmission unit with two compact electric motors, three planetary gear sets and four clutches. This configuration enables the power to be split into an electrical and a mechanical path.
Conventional one-mode hybrid systems tend not to have mechanical ratios. As such, this kind of system transfers the bulk of the power via the electrical path with high electrical output and hence lower efficiency. It works efficiently principally at lower loads and speeds. If power also needs to be transferred at higher loads and speeds, an accordingly larger electric motor would be required. Having to power and find space for this engine would only be possible at the expense of efficiency, according to Daimler.
In response to increasing engine loads and higher speeds, two-mode technology, by contrast, variably shifts the power transfer from the electrical path to the mechanical drive path. As a result, the system is much more efficient in all situations, both in inner-city stop-and-go traffic as well as on fast motorway journeys, while also enabling smaller electric motors to be used. The efficiency boost—both in urban traffic as well as on motorway journeys—is where "two-mode" technology gets its name from.
The four clutches are used to distribute the driving force generated by the electric motor and combustion engine via four fixed ratios and two electric variable ratios to all four wheels on the 4MATIC permanent all-wheel-drive system, depending on the driving situation. Various strategies ensure the best possible fuel consumption and performance, depending on the traffic situation or the driver’s requirements:
When moving off or at low speed up to the second fixed gear, the ML 450 HYBRID operates in ECVT 1 mode (Electric Continuously Variable Transmission), delivering a very comfortable, smooth ride.
At higher speed from the second fixed ratio, ECVT 2 mode is used. Depending on the optimum operation point, a combined or continuously variable mode is selected.
In the first and third fixed gear both electric motors work synchronously, providing either additional energy for acceleration (boost) or storing the braking and coasting energy (recuperation).
In the second and fourth fixed gear, one electric motor is switched off to improve efficiency. The second electric motor is used for the booster function and recuperation.
The two compact electric motors feature a high power density and are fully integrated into the transmission housing. The limited installation space means the two electric motors have to be configured differently. The first electric motor—positioned nearer to the combustion engine—develops 62 kW (85 hp) and 235 Nm (173 lb-ft) of torque and has been designed with power output and minimal losses firmly in mind. The electric motor placed at the end of the transmission develops 60 kW (82 hp) and 260 Nm (192 lb-ft) and is the solution for moving off solely under electric power and providing the booster function.
The electrical energy in the ML 450 HYBRID is stored in a compact, 2.4 kWh, 288V liquid-cooled high-voltage NiMH battery pack. The pack comprises 24 modules (240 cells). A battery management function monitors and controls the various charge flows during electric motoring, boost or recuperation to ensure a stable charge state.
Operation of the two external rotor magneto motors and the management of the entire high-voltage onboard network requires complex control electronics housed in the engine compartment of the ML 450 HYBRID. Waste heat from the electronics is dissipated via a separate cooling circuit to increase efficiency. An integrated Auxiliary Power Module enables power to be exchanged between the 288-volt battery and the 12-volt onboard network.
A computer controls the hybrid system with up to 50 million computing operations a second. The electronic control unit calculates the most efficient operating mode 160 times a second, thus ensuring the lowest possible fuel consumption. The management system also supports new functions and comfort/convenience features:
The transmission’s coordinated shift strategies ensure judder-free manoeuvring;
“Chill down” mode enables the battery to recover during overload or high temperatures;
An adaptive drive mode learns the driving style of the driver or the particular route profile to optimize the agility of the drive mode;
The “Power Launch Control” supports optimum acceleration in boost mode;
The “Traffic Jam” feature enables the vehicle to be driven entirely under electric power in stop-and-go traffic even with a low battery charge status.
Consumption-optimized Atkinson V6 engine. The modified 3.5-liter V6 gasoline engine has been optimized in accordance with the Atkinson process, delivering substantially better fuel consumption and emissions figures with an output of 205 kW (279 hp) and a maximum torque of 350 Nm (258 lb-ft).
The development engineers utilized some of the benefits of the Atkinson principle whereby the expansion phase is longer than the compression phase. The intake valve is kept open slightly longer between the intake and compression phases, which improves the engine’s thermal efficiency while reducing the specific fuel consumption and untreated emissions.
This process is not used with conventionally powered vehicles because Atkinson engines—apart from the lower fuel consumption and emissions—tend to deliver lower torque at lower revs and thus inferior response particularly at low engine speeds. Hybrids offset this drawback through the use of the electric motors when moving off normally, accelerating or using the booster function.
Compared with the conventional 3.5-liter V6 gasoline engine, the Atkinson version features a modified cylinder head, pistons and camshaft. The camshaft timing has also been modified. The newly developed regulated oil pump adjusts the flow rate to the engine’s actual requirements, thus minimizing the necessary delivery rate and helping reduce fuel consumption and emissions. Since the alternator and starter functions are integrated in the two electric motors, these two components, which are now surplus to requirements, no longer need to be connected up to the belt drive on the combustion engine. The air conditioning compressor and the steering servo pump are another two components that do not need to be powered via the belt drive. These are driven electrically on the ML 450 HYBRID and can be run independently of the combustion engine.
Operating strategies of the two-mode hybrid drive. The ML 450 HYBRID offers the full range of extra functions offered by a hybrid drive. These include variable hybrid mode for city and motorway driving, silent, purely electric moving off and motoring, the booster function for fast acceleration, recuperation and the start/stop function. These functions are activated depending on the operating state and traffic situation:
While at a standstill, the combustion engine is switched off. The air conditioning and power steering functions are fully retained thanks to the electric drive of the air conditioning compressor and the steering servo pump.
Manoeuvring, for instance when parking, only uses the electric drive.
If the driver presses the accelerator from a standstill, the ML 450 HYBRID moves off solely using electric power. If the accelerator is pressed rapidly, the management system recognizes the need for increased acceleration, activates the combustion engine which it backs up with additional torque from the electric motor (booster effect). This noticeably increases the vehicle’s agility without having to use the kickdown feature.
When cruising, the operation point is shifted toward lower specific fuel consumption figures to increase the efficiency of the V6 engine while in turn also increasing efficiency particularly under partial loads.
Recuperation, the conversion of braking energy into electric power and subsequent storage starts immediately the vehicle is in overrun mode or during normal braking manoeuvres. The service brake is also activated during heavy braking.
The start/stop function deactivates the V6 engine as soon as it is not needed, say when rolling up to a red traffic light. As soon as the driver presses the accelerator, the ML moves off using electric power and then starts the combustion engine.
The Silent Start enables the vehicle to be started without creating any noise.
A specially designed display concept enables the driver to keep track of the particular drive mode. The system display can be used to either display the current energy flow between the high-voltage battery, combustion engine and electric motors including the percentage charge state of the battery, or a fuel consumption and recuperation energy graphic. The battery charge state can also be called up in the instrument cluster. The conventional rev counter is replaced by a display which illustrates the combined drive system and recuperation output at a given moment.
The ML 450 HYBRID comes with a Sport mode, which is activated using the “E/S” button in the center console. This operating mode provides the handling of a conventional automatic transmission with eight fixed ratios and very short shift times.
The ML 450 HYBRID is produced at the Mercedes-Benz plant in Tuscaloosa, Alabama, in the United States.