Engine and Transmission

Engine and Transmission

Engine

The Two-mode Hybrids feature an all-new aluminum block 6.0 Liter Engine (RPO LFA, VIN Code 5 or RPO LZ1, VIN Code J). This engine offers Late Intake Valve Closing technology. Closing intake valves after the intake stroke has completed allows reverse flow of the air/fuel charge into the intake manifold. This reduces the effective compression ratio, allowing the expansion ratio to increase while retaining normal combustion pressures. Efficiency is gained because the high expansion ratio delivers a longer power stroke and reduces the heat wasted in the exhaust. The lower effective compression ratio does mean less power, but it has been compensated for by increasing the mechanical compression ratio from 9.6:1 to 10.8:1, and the use of electric motors in the hybrid transmission. This engine also features Active Fuel Management, Variable Valve Timing, and an Electronic Throttle Control system. The engine does not have all the typical accessories on it. There is no generator, starter motor, belt-driven air conditioning compressor or hydraulic power steering pump on it. The LFA engine is not flex-fuel compatible.

The Gen-IV Vortec 6.0L hybrid engine features GM's Active Fuel Management technology (AFM). AFM temporarily de-actives four of the eight cylinders under light load conditions. Working in conjunction with the hybrid control system, it provides significant fuel economy increases under the federal government's required testing procedure, and potentially more in certain real-world driving conditions. Truck owners don't have to compromise on the Vortec 6.0L's outstanding peak horsepower to go farther on a tank of gas.

The Vortec 6.0L for the hybrid application utilizes a variable displacement oil pump to minimize parasitic loss and thus increases fuel efficiency even further. Other Vortec Small Block engines use a fixed displacement gerotor pump, which can use up to 2 horsepower more to operate than the variable displacement pump. A series of vanes rotating in a carrier develops oil flow and pressure based on requirements from the engine. An internal spring/slider mechanism allows the pressure and flow to change in response to what the engine needs to operate everything in the lubrication circuit, from bearings to AFM to VVT.

The Gen IV Vortec 6.0L's industry exclusive cam-in-block variable valve timing (VVT) is included in these hybrid applications, and allows the powertrain system to take advantage of late intake valve closing for greater efficiency. VVT eliminates the compromise inherent in conventional fixed valve timing and allows a previously unattainable mix of low-rpm torque, even torque delivery over a broad range of engines speeds, and free-breathing high-rev horsepower.

Two-Mode Hybrid vehicles utilize a specific aerodynamics package, a hybrid electric transmission, a higher efficiency engine and electrically powered accessories to achieve a fuel economy improvement of 30% combined and 50% city over the base models. Combined engine and transmission output is 369 bhp and 380 lb ft of torque, with 332 hp/367 lb ft coming from the 6.0L V8 Gen IV gasoline engine. The Two-mode Hybrid transmission adds the equivalent of the balance of the output, and in addition it increases the time the engine can remain at peak power during acceleration. The fixed gear ratios are designed to let the most power be used over the most useful vehicle speeds.

The Two-mode Hybrid vehicle is capable of operating in either full electric mode for under 45 km/h (28 mph) light acceleration, or in a blended gasoline/electric mode for higher speed or higher load request situations. These vehicles will not maintain auto-stop above 45 km/h (28 mph). How far or how long the vehicle will remain in auto-stop and operate in full electric mode is dependent upon drive motor battery assembly state of charge, electrical load (A/C, 12V accessory, and EPS operation) and temperature.

Transmission

The heart of the Hybrid system is the Transmission. (RPO M99, Hydramatic designation 2ML70) The Two-mode Hybrid Transmission contains two 60kW electric motors, each capable of producing 272 lb-ft of torque, 3 planetary gear sets, and 4 wet-plate clutches. The hybrid transmission can operate in multiple modes including propelling the vehicle electrically with the combustion engine off (auto-stop). Two continuous ratio modes of operation are attained with the electric motors, one with the combustion engine supplying torque, and the other with an electric motor supplying torque. Since the continuous ratio ability is attained with one of the electric motors, these modes are described as Electrically Variable Transmission modes or "EVT" modes. Three 300 volt A/C cables are connected to each of the two motors, these cables attach to the transmission housing via a rigid conduit around the transmission and are connected to an inverter that drives the motors. For maximum fuel economy, the transmission selector should be placed in Drive not manual mode. Four fixed ratios of 3.7:1, 1.7:1, 1.0:1 and 0.7:1 may be chosen in manual mode. The fixed ratios also let the transmission save battery power that would otherwise be used to control range of the variable ratios to keep the engine in its optimal rev range. Fixed ratios also allow overdrive, for faster overall total speed of the vehicle. Fixed gear changes are accomplished with clutch-to-clutch control, where an oncoming clutch is engaged and an "offgoing" clutch is released in a precise manner to achieve the ratio change. The first-to-second upshift, however, is a freewheeling action, where the second gear clutch engages while the first gear one-way clutch spins freely. This allows a greater degree of smoothness at lower vehicle speeds. The Electric motor/generators in the transmission can be used either to propel the vehicle or to generate power to charge the drive motor battery assembly. The transmission control module (TCM) is located inside the transmission.

All vehicle movement in reverse is performed by the electric motors in the transmission. In order to maintain the proper charge in the drive motor battery assembly, the engine will run when the vehicle is in reverse, however, there is no mechanical connection between the engine and the drive wheels in this range. 2010-2012 models are designed to operate in reverse without the engine running, depending upon HV battery state of charge.

This transmission has no torque converter. There is a fluid-filled torsional dampener located between the flexplate and the input to the mechanical transmission fluid pump. This transmission has an auxiliary transmission fluid pump, which is located external and below the bellhousing. It is controlled by the auxiliary pump controller, which is mounted below the 12V battery on the left side of the engine compartment. The aux pump operates during auto-stop and provides transmission fluid to the electric motors for cooling and to maintain clutch pressure. Due to the heat generated by the 60Kw electric motor/generators, there is an auxiliary transmission cooler that is specific to this application.

DEXRON(R)- VI is used as a lubricant and to cool the electric motors. For normal use, there is no fluid change scheduled. DEXRON(R)-VI was developed to have a more consistent viscosity profile; a more consistent shift performance in extreme conditions; and less degradation over time.

The Two-mode Hybrids are offered in both 2WD and 4WD configurations. The transfer cases in the 4WD models are identical to transfer cases found in the base model application. Recreational or Dinghy towing is only permissible on 4WD vehicles with the transfer case in neutral and the transmission output shaft held stationary in Park.

Caution
Under no circumstances can a 2WD vehicle be dinghy towed or the transmission output shaft be allowed to turn. Wrecker towing is to be done by flatbed carrier only.