Since 1999 when hybrid vehicles were introduced in the United States, they have gotten a bad name in terms of their performance. These vehicles have typically been recognized for their ability to get better than average fuel economy, but many automotive enthusiasts consider them to be slow and underperforming. In fact, many see them as having been designed solely for “tree huggers” that are only interested in saving fuel. Given the reputation these vehicles have developed, would you believe that hybrid technology actually has the ability to increase the rate of acceleration? Would you believe that this technology has been used in F1 race cars for going on seven years? This article will focus on the following:
- Why hybrids on the road are typically driven slowly
- Why hybrid vehicle technology can actually increase the rate of acceleration
- How hybrid technology is being implemented into race cars
So, why do people drive hybrid vehicles so slowly? These vehicles are driven slowly so consistently that most people assume it’s that the vehicles are incapable of accelerating quickly. The reality however is that the vehicles are actually coaching drivers to accelerate slowly, not that the vehicles are incapable of going fast. This driver coaching is designed to help the driver achieve the advertised fuel economy. The coach in this case comes in many different formats, but they all have one thing in common. The commonality is the fact that they somehow “reward” the driver for operating the vehicle in a fuel efficient (slow) manner. One example of this coaching was on the Ford Fusion hybrid. Ford chose to display an image of a digital plant in the dash display. The more efficiently the driver operated the vehicle, the more leaves the digital plant grew. I tend to think of these driver coaching systems as a type of video game. As with most video games, they are designed to draw the drivers in and subconsciously get them to play the fuel economy game.
What a Prius and A Quattro E share
You may still be reluctant to believe that hybrid technology has the ability to increase vehicle performance so let’s take a closer look. To begin with let’s look at the performance limitations of traditional internal combustion engines (ICE). We all know that a traditional ICE doesn’t create maximum torque immediately when the accelerator pedal is depressed. In fact, for most vehicles there is a significant delay while the engine speed increases to a peak performance RPM. We refer to this as the ramp up of power as the engine’s torque curve. For an ICE this torque curve looks somewhat like a mountain where it starts low, slowly builds up to a peak, and then drops back off. So, how can hybrid technology improve this? The key to this performance puzzle is the electric drive system in a hybrid. Electric motors perform much differently than an ICE in terms of the torque curve. Much like an ICE, the torque curve of an electric motor is dependent on the overall design. The difference however is that, in general terms, electric motors produce high levels of torque beginning immediately from 0 RPM. In fact, for most electric motors used in hybrid vehicles, the torque available starting at 0 RPM will be near the maximum possible torque the motor can produce.
These unique electric motor torque characteristics can be used to supplement an ICE. When a driver of a hybrid vehicle requests maximum acceleration (wide open throttle acceleration) the vehicle controllers begin a blending of power. The typical sequence will involve the engine controller beginning to ramp up the ICE torque. As previously mentioned, it will take some time for the ICE to reach peak torque output. To improve acceleration during this ramp up of the ICE, the controllers will be commanding additional torque from the electric drive system of the hybrid. Because the electric motors begin producing torque almost instantaneously, the vehicle will have improved acceleration. Of course, there are limitations to all systems. The electric drive system can’t sustain long term acceleration due to the limited power available (dictated by battery storage capacity). In an ideal situation however, the ICE will have reached its maximum torque production range before the electric drive system runs out of power.
Because of this ability to increase the rate of acceleration, hybrid technology has been slowly making its way into performance applications. Current examples of this trend include the Acura NSX, Porsche Panamera hybrid, Porsche Cayenne hybrid, and the Ferrari LaFerrari. All four of those vehicles are using hybrid technology, and it definitely isn’t just to improve fuel economy. Because the Porsche models have both hybrid and non-hybrid versions they provide the opportunity for an apples-to-apples performance comparison. The Porsche Panamera hybrid for instance has a 0-60 mph time of 5.2s. That is .8 seconds faster than the base model Panamera S. The performance difference is even more pronounced in the Porsche Cayenne. The hybrid version of that vehicle has a 0-60 mph time of 5.4s, while both the base model and the diesel versions of the Cayenne have much slower 0-60 mph times of 7.3s and 7.2s respectively.
The performance applications for hybrid technology however don’t stop with consumer vehicles. Race teams have been implementing hybrid technology for several years, and the latest versions of these systems are using some unique applications of this technology to improve performance beyond what is being done in the consumer market. Formula 1 (F1) racing is a perfect example of this.
I’m not an F1 expert by any means so this article will not go into the details of the rules and regulations that have been put into place specific to hybrid vehicle technology. If you’d like to get details on those I’d recommend visiting the F1 website (www.formula1.com) or searching the internet for “F1 ERS rules.” Instead, I’ll focus on the types of hybrid technology that has been used in the F1 cars and how that has transitioned over the years.
Hybrid technology made its initial appearance in the F1cars as something known as KERS. This began as an option during the 2009 race year, and then became mandatory in all F1 cars for 2010. KERS is an acronym that stands for Kinetic Energy Recovery System. While KERS wasn’t required to be an electric drive system, most of the F1 teams chose to go that route. In fact, many of the teams utilized a system from the same supplier (Magneti Marelli). The initial KERS system was relatively simple in concept. It involved an energy storage system (typically a lithium based battery or a capacitor setup) and an electric drive system. The electric drive system contained a motor/generator, an inverter, and the required controllers. These components are all very similar in function to those used in consumer hybrid vehicles.
In the F1 KERS, the motor/generator assembly was connected to the driveshaft. During braking events the KERS unit was able to serve as a generator. Kinetic energy from the vehicle was transferred through the drivetrain to the motor/generator. The transferred energy caused the motor/generator’s rotor to spin which created electricity. This same concept is used in consumer vehicles and is known as regenerative braking. There are two major gains in using this method to slow the vehicle down. First, rather than wasting the kinetic energy by turning it into heat (as traditional friction brakes do) it allows a portion of that energy to be captured for later use. Second, because not as much heat is being generated during braking, the brakes don’t get as hot.
During acceleration events the KERS unit was able to serve as a motor. Energy that had been stored during a braking event could be used to power the electric motor which would work along with the ICE to accelerate the vehicle. While the controls of this system were obviously fairly complicated, the overall design was pretty straight forward. A form of this KERS system continued to be used in F1 through the 2013 race year.
For the 2014 race season F1 cars were required to make changes to the KERS system. In fact, for 2014 the name of the system changed to ERS (Energy Recovery System). The “K” was dropped because the system no longer focused just on recovering kinetic energy. The new ERS system was designed to capture both kinetic and heat energy from the vehicle. As you likely already know, internal combustion engines are very inefficient because a large percentage of the energy from the fuel they burn is wasted as heat. One way to help recover some of that lost heat energy is through the use of a turbo charger. The ERS system was designed to further improve the recapture of that lost heat energy beyond what a turbo charger can do by itself.
The ERS utilizes two motor/generator assemblies instead of the single one that was used in the KERS. One motor/generator assembly called the MGU-K (Motor/Generator Unit-Kinetic) serves a similar purpose to the previous KERS. The MGU-K is used to capture kinetic energy from the vehicle, store it in an energy storage system (typically a battery, capacitor, or a combination of the two), and deliver additional torque to the wheels when needed. The real improvement in the ERS design revolves around the addition of the MGU-H (Motor/Generator Unit-Heat). The MGU-H is used to capture heat energy from the exhaust in the form of electricity. That electricity can be used power the MGU-K to provide additional torque to the wheels, or it can be stored in the energy storage system for later use. To be able to capture the heat energy from the exhaust, MGU-H in the F1 system is connected directly to the turbo. When the MGU-H rotor is spun by the turbo it serves as an electrical generator.
MGU-H goes one step further than just serving as a generator though. In fact, it is actually capable of improving the efficiency of the turbo. As you likely know turbo charged systems have limitations. Because a turbo charger is driven by the engine exhaust there are times when it doesn’t produce enough air volume to achieve the ideal intake pressure, and times when it produces too much air volume. When aggressive acceleration is requested from a low RPM, a turbo charger typically can’t produce the desired volume immediately. This “turbo lag” is due to the time it takes for the exhaust gases to spool up the turbo after the accelerator pedal has been depressed. Turbo lag is one form of efficiency loss in turbo charged engines. During high RPM operation with the engine under high load, the turbo charger has the ability to produce more volume than is necessary. The excessive air volume from the turbo can cause intake pressures that exceed acceptable limits which could cause engine damage. To prevent the potential for an over pressure condition in the intake, traditional turbo charger applications are fitted with a waste gate. The waste gate is used to discharge the excessive pressure which prevents engine damage, but also results in wasted energy.
In the F1 ERS, the MGU-H can help reduce both of these losses. First, because the MGU-H is connected directly to the turbo it can be used as a motor to increase turbo speed. Stored electricity is delivered to MGU-H allowing it to accelerate the turbo speed much faster than exhaust gases alone could. This helps reduce, or possibly even eliminate, “turbo lag”. When the turbo charger is producing more volume than the engine requires, the MGU-H can be utilized as a generator. This prevents the need to bleed off excess pressure through a waste gate, and instead provides electrical energy that can be utilized immediately by MGU-K or stored for later use. Think of this as regenerative braking for the turbo where the braking effect is applied at just the right amount to maximize turbo efficiency.
The changes seen in F1 over the last several years are not unique. In fact, the governing body for the annual 24 hours of Le Mans race issued new rules in 2014 related to vehicle design. Those rules included the requirement for all factory teams competing in the premiere Le Mans Prototype 1 category to utilize a hybrid-electric drive system. So while I don’t expect we’ll be seeing hybrid-electric technology in a NASCAR race any time soon (at least not beyond the pace car) these applications prove that hybrid vehicles don’t have to be slow.