When the 2011 Honda CR-Z hits the streets this Fall, it will feature the latest iteration of the company's Integrated Motor Assist (IMA) mild hybrid system. The basic concept of IMA hasn't changed since the original Insight debuted in 1999, but it has been refined to improve performance and efficiency while reducing overall cost.

There's a wide spectrum of hybrids in the market, ranging from low-end, belted-alternator-starter (BAS) mild hybrids to the full-blown power-split strong hybrids at the top. The latter category includes systems from Toyota, Ford and General Motors (two-mode), while Honda's IMA provides a healthy boost in overall efficiency compared to a conventional powertrain, but falls well short of the best power-splits. Read on to learn more about the IMA system and how it's implemented in the CR-Z.


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Architecture

The basic architecture of IMA consists of a compact electric motor sandwiched between the internal combustion engine and transmission. A nickel metal hydride battery pack is used for storing the energy captured through regenerative braking. The final component is the power electronics module that incorporates the inverter, battery management system and motor control.

When the first generation Insight launched in 1999, it was only available with a manual gearbox until a continuously variable transmission (CVT) was added in 2001. The manual was discontinued around 2003 and all subsequent Insight and Civic hybrids were only available with the CVT. The short-lived Accord Hybrid used a conventional step-ratio five-speed automatic. The introduction of the CR-Z marks the return of a manual transmission, this time with six forward ratios, with a CVT as an option.

Motor

The motor is integrated into a 61 millimeter wide case that's bolted directly to the output face of the engine. From the beginning, IMA has used a permanent magnet AC synchronous motor which is both compact and efficient. The stator is fixed to the internal perimeter of the case while the rotor is bolted to the output face of the crankshaft. The clutch for the manual transmission or the input plate for the CVT are bolted to the output side of the rotor.



The electric current in the adjacent coils of the stator is flowing in opposite directions, creating reverse magnetic fields. The motor control causes the current direction (and thus the field) to switch back and forth, which causes the rotor to move in response. The switching rate controls the motor speed and as the rotor spins, it applies drive torque to the transmission providing an electrical boost.

Since the motor is hard-coupled to the engine, there is no mechanism to shut-off and de-couple the engine and drive on electric power alone the way strong hybrids like those from Toyota and Ford. Starting several years ago with the Civic Hybrid, Honda added the ability to shut off fuel flow and close the valves with the VTEC variable valve timing system when cruising at low speeds. This allows the car to motor along on just electricity. However, the hard-coupling means that the crankshaft is still turning and the pistons are pumping. The result is more drag than you would find in a strong hybrid. Because of the sportier nature of the CR-Z, the VTEC system has been used to increase power rather than boost efficiency, so it can't motor along on electrons alone.

At 13 horsepower and 58 pound-feet of torque, the motor (which is shared with the Insight) is adequate for providing a noticeable boost in performance, but not really sufficient to propel the CR-Z on its own for any significant time or distance. However, the beauty of an electric motor is that it typically produces maximum torque at zero speed and stays at that peak before dropping off, making it ideal for boosting off-the-line performance without sacrificing fuel consumption.



Engine

While Honda's inline-four cylinder engines are highly regarded for smooth and efficient operation, low-end torque has rarely been considered a strong suit. This is certainly true of the 1.5-liter unit in the CR-Z with its 113 hp and 107 lb-ft peak torque, especially since that peak occurs at a fairly lofty 4,800 rpm. At 1,000 rpm, the engine is producing barely 80 lb-ft. The addition of IMA has allowed powertrain engineers to fatten up the bottom end of the net torque curve so that it produces 128 lb-ft of torque between 1,000 and 1,750 rpm on the six-speed manual-equipped model. On the flip side, the CR-Z with the CVT is limited to 123 lb-ft of torque. Regardless of the transmission choice, total combined peak power of the engine and motor is 122 hp at 6,000 rpm.

In addition to providing a torque boost during acceleration, the IMA motor is also used to start the engine when you twist the ignition as well as re-starting it after an idle-stop. Generally, any time the car comes to a stop, the engine will shut-off to conserve fuel and cut emissions. However, if the battery state of charge is too low or the electrical demands are too high (when driving with the headlights on or with the air-conditioning turned up) the engine won't be shut off. On the manual transmission CR-Z, the engine is only halted if the car is stopped and the transmission is in neutral. When the clutch pedal is pressed or the brake released, the engine automatically re-starts.

Regenerative Braking

Like other hybrid systems, applying a drive torque to the rotor turns the motor into a generator. During deceleration, the motor is used to recharge the battery and slow the vehicle, negating the need for the brakes to be used as much. The low cost nature of IMA hybrids means that Honda doesn't incorporated fancy brake-by-wire systems to blend friction and regenerative braking. Instead, the regenerative braking is overlaid on the friction brakes and then gently ramped out at low speeds. Compared to the Civic Hybrid, which exhibits a distinct loss of deceleration at about seven mph as the regenerative braking is switched off, the Insight and CR-Z have a much more gradual phase out making the transition almost imperceptible.



Battery

The key to any hybrid, whether it's a mild or strong system, is the energy storage system. Like every other system on the road, the CR-Z uses an electrochemical battery; in this case, a nickel metal hydride unit. The 100.8 volt unit consists of 84 "D-sized" cells arranged in seven modules of 12 cells each. It has a total capacity of 580 watt-hours, a bit less than half the size of most strong hybrids. Like the motor, the guts of the battery pack in the CR-Z are basically carried over from the Insight. The most notable change for the battery is the air cooling system which has been upgraded to improve its efficiency. The cell temperatures are closely monitored and controlled closer to the optimum operating temperature.

Batteries have a limited temperature range where they can provide optimal performance both for charge and discharge. The new cooling system is better able to manage the battery, especially at higher ambient temperatures which improves the ability to recapture and release energy when it's hot. Tighter control of the temperature also improves the durability of the battery pack.



Sport and Econ modes

In keeping with the CR-Z's mission of being a sporty hybrid, Honda has also added a driver selectable sport mode in addition to the Normal and Econ modes found in the new Insight. The Econ mode filters the driver commands and slows down the throttle response to smooth out acceleration and improve efficiency. The new Sport mode does the opposite, cajoling the CR-Z to life. Both the manual and CVT versions receive reduced steering assist and increased throttle response in Sport mode, while the CVT chooses a lower gear ratio range for higher engine speeds and more available power. With the manual transmission, the system monitors the accelerator pedal position and if the driver applies the gas beyond a certain rate, the IMA will provide full motor power immediately rather than ramping it up.

Given the limited hardware capability of the IMA system, Honda has managed to do quite a bit with it. While Toyota arguably created the first performance-oriented hybrid systems with the Lexus GS450h and LS600h, Honda is the first to create a dedicated platform and body that is overtly sporty. The CR-Z is by no means a speed demon, as you've read in our driving impressions, but it certainly feels much stronger than the Insight.

Fans of the original CRX of the late-80s may complain that the hybrid CR-Z can't match the 51 mpg combined rating of the old HF. However, it's also important to remember that the way fuel economy sticker values are computed has changed several times in the last 25 years. The EPA estimates that under the current procedures the CRX HF would have been rated at 43 mpg. The CVT version of the CR-Z is rated at 37 mpg combined which is lower than the CRX but the new car is considerably larger and heavier. The CRX would never meet current emissions or crash safety standards and it's also considerably slower than the modern car. In almost every way, the CR-Z is a much better car. But is it really a better drive? Find out here.


Related Gallery2011 Honda CR-Z Powertrain