By Jim Kerr

Photo Gallery:
2008 Chevrolet Tahoe Hybrid

GM’s new Two-mode hybrid powertrain will be available first in its full size sport utilities: the 2008 Chevrolet Tahoe Hybrid and GMC Yukon Hybrid started production this spring and will be sold in select dealer showrooms. I say “select” because not every dealer that wants one will be able to get one. The demand for this new hybrid vehicle is already high and the initial production is limited to only some dealers. As production increases, other dealers will receive them too. Here is why they have been in high demand, even before production started.

First of all, this is a full size SUV available with four-wheel drive, but it gets fuel economy better than a V6 equipped Toyota Camry. It also gets 32% better fuel economy than a non-hybrid 4×4 Tahoe or Yukon. This hybrid doesn’t skimp in the power department either. A 6.0-litre V8 combined with a two-mode electric/mechanical automatic transmission can tow a 6000-lb trailer.

General Motors two-mode hybrid transmission. Click image to enlarge

Here’s how they do it: the two-mode transmission is key to the hybrid system. This transmission contains three planetary gear sets and four clutches that give the transmission four mechanical speeds. This is combined with two 300-volt electric motor/generators inside the transmission case that are capable of driving the vehicle by themselves up to 41 km/h, or in combination with the gasoline engine. The electric motors are also used to start the gasoline engine, which eliminates the conventional starter system and reduces vehicle weight. Like other hybrids, this SUV also operates in Autostop mode where the gasoline engine shuts off when operating conditions are right and the vehicle is at a stop. As soon as power is required from the gasoline motor, it is instantly started again by the transmission electric motors.

Instead of a 12-volt battery under the hood, the Tahoe and Yukon hybrids have a battery pack under the second row seats. With the seats folded, the flat floor above the battery pack still allows excellent cargo capability and access to the third row seats. Inside the battery pack, there are 40 individual nickel metal hydride battery modules, each putting out 7.2 volts. To produce the voltage necessary to drive the motors, the 40 modules are connected in series to produce about 288 volts DC. This voltage is supplied through relays that disconnect the battery pack during vehicle shut down and then out to the drive motor/generator power inverter module. This module controls power to the transmission motors as well as converting the voltages to 14 volt and 42 volt levels to operate vehicle lights, accessories and electric motors for power steering, brakes and transmission pressure.

General Motors two-mode hybrid transmission electric motors. Click image to enlarge

The power inverter module also controls charging of the batteries. This can be done during regenerative braking or they can be charged by the gasoline engine driving the electric motor/generator units during cruise conditions. Because there is a lot of heat generated in the power inverter module, it has its own cooling system separate from the gasoline engine cooling system. There is a heat exchanger (radiator) at the front of the vehicle and electric pumps to move the coolant through the system. De-ionized water and Dexcool are mixed to provide the coolant and it is important that regular tap water not be used in the system because it can cause corrosion on the switching transistors inside the power inverter module.

The 6.0-litre Generation IV small block V8 engine has been specially modified to work with the hybrid electric motors. This engine features Active Fuel Management, which deactivates four of the engine’s cylinders during light load operation to save fuel. Variable cam timing is computer controlled to provide both low end torque and high speed power, but this engine also keeps the intake valve open a little longer than a normal engine, allowing a reverse flow 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. Engine efficiency is improved but at the expense of some power but this is compensated by increasing the engine compression ratio and supplementing low end power with the power from the electric motors in the transmission.

The hybrid powertrain is only part of the fuel economy story. Aluminum body panels, lower profile hood and fenders and reduced aerodynamic drag all help keep fuel in the tank and money in your wallet.