August 28, 2002

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Honda Civic Hybrid powertrain
by Jim Kerr

Lift the hood of Honda's latest Civic model and you will find something different. A hybrid gasoline/electric engine and CVT transmission replaces the conventional powertrain. With room for 5 passengers and acceleration comparable to other Civics, this Hybrid achieves excellent fuel economy of 4.6 litres per 100 kilometres on the highway. Let's see how this is accomplished.

Part of the hybrid's power comes from a 1339 cc four cylinder gasoline engine. There are many features designed into this engine to increase economy, such as low friction pistons with micro-dimples to increase lubrication and offset cylinder bores to reduce piston side thrust friction. Two new features play a big part in the increased economy: VTEC cylinder idling system, and Dual Sequential ignition.

You may be familiar with Honda's VTEC engines where at higher rpm the valve lift and opening duration are increased by switching operation to a different cam profile. The hybrid's VTEC system is much different. The Hybrid's single overhead cam four-cylinder engine uses the VTEC system to close the valves on up to three cylinders during deceleration. This allows the pistons to move easier because there is no air in the cylinders and engine braking is reduced.

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Normally, drivers want engine braking during deceleration so the brakes don't have to do all the work, but on the Civic Hybrid, the engine braking of the gasoline engine is reduced so the deceleration torque can be directed to the electric motor/generator to help recharge the battery pack. When the gasoline engine rpm drops to near 1000 rpm, the VTEC system switches the valves back into operation for smooth idle. It sounds complicated but works so smoothly you don't even know it happens.

The Civic Hybrid also uses Dual Sequential Ignition (DSI) with two sparkplugs for each cylinder to optimize performance and economy. Each of the plugs can be fired individually. When the air/fuel mixture enters the cylinder, the sparkplug closest to the intake port ignites the mixture and then shortly thereafter the second plug fires to speed up the combustion process. This results in more complete combustion than a single plug system. The timing of the sequential ignition varies with throttle position and engine rpm. For example, at half throttle and low rpm, the sparkplugs fire sequentially, while at half throttle and mid to high rpm the plugs fire simultaneously. At full throttle and medium rpm, the plugs again fire sequentially to balance power and maximise fuel economy.

The electric motor is a marvel of engineering. It acts as the starter motor for the gas engine, the alternator to charge the batteries, and as an electric motor to assist the gas engine during acceleration. Honda calls it Integrated Motor Assist (IMA). The motor is built as part of the engine flywheel and is only 65 mm wide. The only clue to its location is the bright orange power cable going to the transmission.

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Combined with the gasoline engine, they produce 93 HP at 5700 rpm and 105 lb.ft. torque at 3000 rpm. The two work together so well that unless you are watching the fascinating Charge/Assist dashboard display, it is difficult to notice any difference from a traditional automobile!

Honda took the IMA computer controls, battery pack, and cooling system from their Insight Hybrid back to the drawing board and downsized it by 42% into one Intelligent Power Unit module. This allows it to sit vertically behind the Civic's rear seat back. The battery pack receives much of its charge during deceleration, but if it becomes too low, the gasoline engine can also turn the electric motor to charge the battery.

Coupled to the Hybrid's powerplant is a CVT (continuously variable transmission). The CVT works by varying the position of a steel belt between two metal pulleys. Computer controlled, the pulleys use hydraulic pressure to change their width, providing an infinite range of gear ratios. It operates like a sophisticated snowmobile. With the shifter in Drive and light throttle, you probably wouldn't notice anything different other than it seems very smooth. Step harder on the gas and the tachometer goes up to between 4000 and 5000 rpm and stays there while you accelerate. You can hear the engine but the sound doesn't change much. If you didn't look at the speedometer, your ears might deceive you into thinking this Hybrid wasn't accelerating, but it is, and quicker than you expect.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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