Ford Fusion Hybrid: Toward an Integrated Hybrid Future
There are two frequent misconceptions about hybrid-electric vehicles. One is that the slow ramp-up of hybrid sales means that future hybrid sales also will be limited. The second is that the major challenges facing hybrids are related to the cost of the additional hardware.
The reality is that hybrids are as much software as hardware. Sure, the hardware is important and future cost reductions are required for the mass market to accept hybrids. But these hardware cost reductions are coming. The most obvious is the recent development of better lithium ion batteries. These high-power batteries are perfect for conventional hybrids. They will deliver the high power needed to assist with acceleration, run the vehicle occasionally on the battery pack, and capture high rates of regenerative braking energy with a much smaller—and cheaper—battery. Major cost reductions also are available from the electric motor and the power electronics. While electric motors and electronics have been around for a while and are mature technologies, the automotive demands for very high power from compact systems is unique. New electric motor designs and high-power electronics were developed for hybrid applications; substantial cost reduction can be expected with further learning and economies of scale.
But you can’t just throw a lot of money at hardware and expect a hybrid vehicle to work properly. Hybrids gain their efficiency advantage from shutting off the engine at idle, capturing energy that would otherwise be lost to heat in the brakes, enabling engine downsizing by assisting with acceleration, and optimizing the operation of the engine. All of these features require careful calibration and integration of both the electric system and the engine. Even idle-off requires numerous calibration considerations. For example, the restart must be fast and smooth, the engine shut-off must not affect the deceleration rate of the vehicle, and the engine must remain running in cold weather while it is warming up.
The other strategies are far more complex. The electric motor generates electricity during regenerative braking by slowing down the vehicle. If the vehicle slows down faster or slower than the customer expects, or if the braking rate changes without any input from the driver, you will not have a happy customer. Engine downsizing is a great way to improve efficiency, but it requires that the engine and electric motor be operated together to maintain performance. The hand-off in power from the engine to the motor and back again, and the use of both simultaneously, must be smooth and proportionate to the throttle movement.
Even these basic functions of the hybrid system are difficult to calibrate and coordinate. And there are trade-offs involved. For example, efficiency can be improved by using a little more regenerative braking than the customer expects, with the trade-off of nonlinear brake pedal feel. Drivability problems can be fixed by less aggressive transfers from engine to motor and motor to engine power transfers, with the trade-off of reduced efficiency benefits.
These functions are just scratching the surface of the potential of hybrid systems. The real benefit of hybrids will come from optimizing the operation of the engine and the transmission, such that engine operation in low-efficiency modes is minimized and the engine is operated in its highest efficiency modes. For example, using the electric motor to assist with moderate acceleration actually reduces the efficiency of the engine, as the engine is more efficient at high loads and using motor assist reduces the load on the engine. Optimum efficiency during acceleration is obtained by operating the vehicle on the motor alone for low-speed, mild accelerations where the engine is inefficient, on the engine alone when the load exceeds the motor power, and on both only when the load exceeds the engine power. Making these transitions smooth and seamless will be far more difficult than current hybrid calibration. Another example is the dual-clutch automated manual. This transmission offers fast, smooth shifts without torque interruption and much better efficiency than current automatics. But it has problems launching a vehicle from a stop and, thus, requires efficiency-reducing hydraulic systems to ensure a smooth launch. But if the dual-clutch transmission is paired with a hybrid system, the vehicle could launch on the electric motor and the transmission engaged when it can be done smoothly. This has significant efficiency benefits, but again requires careful calibration.
Normally, calibration is an incremental business. Engineers take what worked well on the previous vehicle and try to add another feature or two on the next vehicle. This ensures reasonable drivability and performance for customer satisfaction. Trying to change every calibration at once can easily lead to unexpected problems and program delays and customer dissatisfaction. Toyota was the first manufacturer to recognize the difficulty in calibrating hybrids. The first-generation Prius was not just a PR stunt. It was also Toyota’s attempt to begin traveling up that learning curve and thereby gain years of a head start in developing good calibration strategies.
Up until now, that strategy has paid large dividends for Toyota. It dominates the hybrid market and the term “Prius” is almost synonymous with hybrids. This makes Ford’s achievement with the 2010 Fusion Hybrid all the more impressive. Somehow, Ford managed to climb the hybrid calibration learning curve very quickly and even surpass what Toyota has managed to do. The Fusion Hybrid has better fuel economy, performance, and drivability than the Camry Hybrid—a truly impressive performance that epitomizes the meaning of the Hermance Vehicle Efficiency Award.