2013 VW Jetta Hybrid's Clever Powertrain Engineering
The basic hybrid recipe is relatively simple: (1) take a four-cylinder gasoline engine, (2) modify it to use the Atkinson cycle (i.e., flowing a small amount of the unburned air-fuel mixture back into the intake manifold through late intake valve closing, decreasing combustion volume, and effectively increasing the engine’s expansion ratio), (3) add an electric motor and batteries, and (4) use a CVT to get the power to the wheels. However, Volkswagen engineers took one look at this and said, “Das ist verrückt!”
Crazy it might be, but it has worked for everyone from Toyota to Ford to Nissan and others, each following the same general recipe. Volkswagen could have done it for the Jetta. Modifying its 2.0-liter four-cylinder engine would have been easy. Taking the work expended on the front-drive Audi A6’s available CVT, and applying it to a smaller unit would have been straightforward. Ditto the procurement of the batteries and electric motor. Plop all the ingredients into a run-of-the-mill Jetta sedan, and you have a relatively inexpensive, ready-to-go hybrid with which you can undercut the competition on price.
Doing so would have tied VW to its recent past, not its future, so it took a different route. The 2.0-liter engine found in the Jetta S wasn’t modified, and the larger 2.5-liter inline five wasn’t even considered, because VW is in the midst of modularizing all of its platforms and powertrains. These two engines won’t be around for long. Instead, it tapped its new turbocharged and direct-injected EA211 family of small (1.2- and 1.4-liter displacement) engines, taking the larger of the two for this application.
Like all of VW’s new engines, gasoline and diesel, the EA211 leans back toward the firewall at a 12° angle. What’s more, the exhaust manifold is at the back and the intake at the front; it retains the 82-mm bore spacing of the EA111 family of engines; and mounting length has been reduced by nearly 2 in. Switching from a cast iron to a die-cast aluminum crankcase shaved off nearly 50 lb., while integrating the exhaust manifold into the cylinder head permitted dual-loop cooling (including a dedicated loop for the intercooler and turbo housing) for faster warm-up and better emissions performance.
The engine is mated to VW’s slick dual-clutch DSG automatic that has a dry clutch. Between the two sits a 27-hp ZF-Sachs electric motor and clutch that allows the engine and motor to be engaged and disengaged from each other. Combined, these units produce 170 hp and 184 lb-ft of torque, enough to push the Jetta Hybrid to an electronically limited top speed of 125 mph. By the way, full torque is available from just 1,000 rpm.
This powertrain layout declutches the
gasoline engine under braking—elimi-nating engine braking when the throttle is closed—and this greater reliance on the brakes increases the regenerative energy available to top off the batteries. (It’s possible to drive on electricity up to 37 mph in regular mode or 44 mph in E-Mode, accessed by a button on the console, for distances of up to 1.2 miles.) This design also decouples and shuts off the engine when the driver lets off the gas at speeds up to 84 mph in order to eliminate the parasitic drag the engine would place on the driveline. Closing the clutch between the motor and engine starts the gasoline powerplant. At freeway speeds, the electric motor is used either to drive the electrical system, or to provide boost for quicker acceleration.
All is not sweetness and light, however, The trunk takes a bit of a hit with the addition of the hybrid powertrain. A 4.2 ft3 hit, to be exact. The reason is that an 80-lb, 220-volt, 60-cell Sanyo lithium ion battery pack is located above the rear axle. It has a rated energy of 1.1 kWh, a pulse power of 32 kW, and creates a sizable “step” in the trunk floor. You still get folding rear seats and a pass-through, but the hump eliminates the flat load floor of non-hybrid Jettas. Thankfully, the battery pack’s cooling system takes up little room. Air is drawn from the passenger compartment into a duct located under the rear seat, sent through a plenum attached to the pack, and out through a duct in the passenger’s side rear quarter panel. It follows much the same path as air vented from the passenger compartment, and helps fill in the low pressure area behind the car.
The Jetta Hybrid has a sealed front air intake to control the amount of air allowed into the engine compartment. Unique side skirts, front air dam and rear spoiler control flow around the body, while additional underbody shields and directional vanes corral the air, directing it to the rear bumper’s diffuser. Interestingly, VW designers took the opportunity to use the diffuser’s close proximity to the fuel tank and muffler to hide the exhaust pipes from view. VW isn’t the first to do this, but it is interesting that automakers feel the need to downplay the combustion engine aspect of their hybrid vehicles.
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