Inside the Honda IMA Hybrid System

Honda has long been a leader in applying advanced automotive technologies to increase fuel economy and reduce emissions. In the gasoline crisis days of the early 1970s, Honda introduced its Civic CVCC with its pace-setting Compound Vortex Controlled Combustion (CVCC) engine technology. A few decades later, Honda was the first automaker in America to have a hybrid electric vehicle (HEV) in its dealers’ showrooms.

The two-seat Honda Insight hybrid electric vehicle debuted in late 1999 as an early 2001 model…quite an accomplishment since, with the exception of Toyota, most of the rest of the industry is just now getting serious about hybrids. In 2003, Honda introduced its second generation HEV, the five passenger Honda Civic Hybrid.

Both Honda models are parallel hybrid configurations – in other words, the wheels are powered by both the internal combustion engine and an electric motor. These two Honda Integrated Motor Assist (IMA) systems feature a smaller-than-normal gasoline engine and a thin, pancake-type electric motor/generator located between the engine and transmission. Their fuel-thrifty internal combustion engines – a 1.0-liter three-cylinder in the Insight and a 1.3-liter four-cylinder in the Civic – provide all the power needed for most driving situations. When additional power is needed, such as when passing or climbing grades, the integrated electric motor/generator performs in ways similar to a supercharger, seamlessly kicking in to supply added power. The motor/generator also functions as a high-speed starter and as a generator for battery charging during regenerative braking.

Unlike the hybrid configurations used by Toyota and Ford, Honda’s hybrids cannot operate solely on battery power as pure electric vehicles under specific driving circumstances. Still, Honda’s IMA fits the longstanding definition of a true integrated full hybrid, designed from the ground-up to enable vehicles to run super-efficiently on shared internal combustion and electric power. Those who cast it as a “mild” hybrid do so incorrectly. The simpler and less costly “mild hybrids” being developed by some automakers typically use an integrated starter/generator that automatically shuts down an engine when a vehicle stops, then seamlessly starts it up when it’s time to go again. The result is a modest improvement in fuel economy of maybe 10 percent, an important achievement in an era where any bump upward in fuel economy is a good one, but a world apart from the 40, 50, and 60+ mpg fuel economy achieved by a full hybrid system like Honda’s IMA.

Honda hybrids achieve these significant fuel economy numbers through several means. Primarily, the motor/generator’s ability to augment internal combustion power allows the use of a smaller displacement engine with a commensurate decrease in fuel usage. Regenerative braking also recoups energy upon deceleration that would otherwise be wasted or simply dissipated as heat during braking. Electricity created in this process is stored in the batteries for use as electric power is needed. Automatically shutting down the internal combustion engine while idling, such as at traffic lights, also saves fuel. Reduction of vehicle weight through use of lightweight materials plus improved aerodynamics and decreased rolling resistance also serve to improve fuel economy.

VARIATIONS ON A THEME

The IMA system used in the Honda Insight features a 1.0-liter, three-cylinder VTEC-E (Variable valve Timing and lift Electronic Control) gasoline engine that produces 54-horsepower. The ultra-thin, permanent magnet, three-phase synchronous electric motor/generator sandwiched between the engine and transmission adds 13 horsepower (10 kilowatts) as needed. This adds up to a total of 67 horsepower, which gives sprightly, if not neck-snapping, performance. The best part is the EPA numbers: 60 city/66 highway miles-per-gallon with the five-speed manual and 57/56 mpg with the continuously variable (CVT) automatic transmission.

The larger Civic Hybrid uses a 1.3-liter, four-cylinder with advanced technologies like VTEC, Dual-point Sequential Ignition (i-DSI), two spark plugs per cylinder, a lean NOx (oxides of nitrogen) catalyst system for emissions reduction, and various friction-reducing techniques. This IMA powerplant capably produces a combined 93 horse-power. Like in the Insight, an ultra-thin, brushless DC motor/generator assists the internal combustion engine. Because of the high torque characteristics of electric motors, especially at low speeds, torque is increased by an impressive 66 percent at 1000 rpm. At an EPA rating of 46 urban/51 highway mpg with a five-speed manual transmission, this sedan gets up to 650 miles on a tank of gas. Civics equipped with a CVT automatic achieve a fuel economy rating of 48/47 mpg.

The latest entry in Honda’s stable is the 2005 Accord Hybrid with its V-6 IMA system, a 240+ horsepower package that provides better performance than the conventional V-6-equipped Accord, with the benefit of four-cylinder Civic fuel economy. To maximize fuel economy, this model also incorporates Honda’s new Variable Cylinder Management (VCM), which deactivates three of the six cylinders under low-load conditions without sacrificing performance.

In most states, Honda’s hybrids are certified as SULEVs, or Super Ultra Low Emission Vehicles. Because of the credits offered toward Zero Emission Vehicle mandates in California and certain Northeastern states, Honda equips its hybrids in these markets with a zero evaporative emission fuel system and additionally warrants the emission control system for 150,000 miles, making these certified Advanced Technology – Partial Zero Emission Vehicles (AT-PZEVs), the cleanest-running category of vehicles outside of fuel cell and battery electric vehicles.

A nickel-metal-hydride (NiMH) battery pack is used in Honda hybrids. The hybrid vehicle battery features stable output characteristics regardless of the state-of-charge status and is also extremely durable, designed to last 10 years under normal driving conditions. The brains of the IMA system is the Power Control Unit (PCU), which precisely controls the motor assist, regenerative braking, and battery charging functions, including both the NiMH battery pack and the conventional 12-volt battery used for lighting and power accessories. Just one example of the PCU’s function is illustrated during the power assist mode, when the PCU determines the amount of auxiliary electric power needed based on throttle opening, various engine parameters, and battery state of charge.

GETTING TECHNICAL
Clearly, that’s enough information for most people to digest. But if you’re a “gearhead” and you must know the specifics about how Honda’s IMA works, read on.



Operating in the motor mode, the IMA motor/generators starts the gasoline engine and instantly spins it up to 1000 rpm. As a back-up, this job can be handled by the Honda’s conventional 12-volt starter if, for example, the battery module state-of-charge is too low, the car is operating in extremely cold or hot weather, or in the unlikely event the IMA system fails. Direct current from the battery module is converted to AC electric power by the motor drive module (MDM). This electricity is supplied to the IMA motor/generator operating in motor mode for accelerating, climbing hills, and other high load conditions. For maximum acceleration, both the IMA motor/generator and gasoline engine are used. Under light acceleration, the motor/generator provides only partial assist in an amount determined by load and throttle position. Once cruising, the Honda hybrid is propelled solely by the gasoline engine. If the battery module state-of-charge is low, some of the engine’s output drives the IMA motor/generator operating in generator mode for recharging. If fully charged, a small output is still used for the vehicle’s 12-volt accessories and battery.

The gasoline engine is switched to the fuel cut mode when slowing down. The IMA motor/generator in the generator mode is driven by the vehicle’s wheels. The MDM converts its AC output into DC power for both battery module charging and the 12-volt system while slowing the vehicle. Partial charging occurs if brakes are not applied, while applying the brakes results in greater deceleration and electricity generation. Regeneration continues until engine speed drops to about 1000 rpm and when the transmission, either manually or automatically, is shifted into neutral. The engine will be shut down if the battery module state-of-charge is sufficient, although the engine may idle for a short time before stopping if it’s not clear that a stop is imminent. The engine will continue to run at a fast idle if battery charging is required.



Additionally, the gasoline engine is shut off when propulsion power is not needed unless air conditioning is required. The engine automatically stops when vehicle speed is below 19 mph and brakes are applied, or when speed is less than 3 mph. It will also stop when the transmission is in any gear, except first, before slowing down, or when the transmission is in neutral and engine speed drops below 1000 rpm. The engine is restarted when the accelerator is applied again, a gear is selected with the clutch disengaged on manual transmission vehicles, the brake pedal is released during deceleration, or the battery module’s state-of-charge drops below a threshold level. Idle stop will not operate if the engine has not yet warmed up, the transmission is in reverse, the battery module is not sufficiently charged enough for the IMA motor/generator to restart the engine again, or the system detects stop-and-go traffic conditions.

That’s Honda’s IMA in its smaller displacement versions as we know it today, although the latest V-6 IMA variant operates somewhat differently (a story for another time). It’s always possible that Honda could add a twist or two with a next-generation hybrid system, but with its Integrated Motor Assist powerplant providing such an admirable coupling of performance and fuel economy, it isn’t likely anytime soon. Plus, the IMA powerplant has proven to be quite scaleable, so expect to see it widely used on other Honda and Acura models in the future.