Honda's Compact & Clever New Hybrid Powertrain
The powertrain for the 2003 Honda Civic Hybrid is small. And by this we mean the internal combustion engine, transmission and the 10-kW electric motor/generator that's fitted between the two.
As for the engine, it is bigger than the one used in the 2002 Honda Insight Hybrid. The Insight uses a 1.0-liter, three-cylinder engine that produces 67 hp @ 5,700 rpm. The Civic Hybrid uses a 1.3-liter, four-cylinder engine that provides 85 hp @ 5,700 rpm. By way of comparison, the 2002 Civic LX sedan is equipped with a 1.7-liter, four-cylinder engine that produces 115 hp @ 6,100 rpm.
Dimensionally, the Civic Hybrid's Integrated Motor Assist (IMA) package—and it should be noted that this is the second generation IMA, with the first being put in the 1999 Insight—is small, too. That is, compared to the 1.7-liter engine used in conventional Civics, the 1.3-liter engine is shorter, thereby permitting the transversely mounted engine, electric motor/generator, and transmission (manual or continuously variable [CVT]) to be fitted in the same amount of engine bay space. Contributing to this is the use of what's described as an "ultra-thin" DC brushless electric motor. The 13-hp, 144-volt motor is just 2.55-in. wide. One way that this motor is able to be small but powerful is through the use of an asymmetrical wire winding method rather than the symmetrical approach that's ordinarily used when winding motors. The asymmetrical cross section contributes to a 27% improvement in space efficiency.
The Civic Hybrid has an aluminum block with thin-sleeve construction. Which simply means that the bores are close together. In order to reduce the friction between the cylinder walls and the aluminum alloy pistons, plateau honing is used on the walls. This is a two-stage process rather than the single-stage honing that is more common in engine manufacture. Also minimizing friction is an offset cylinder bore design: the crankshaft is aligned such that the side thrust of the piston is minimized just after the top-dead-center position in the combustion cycle.
Speaking of combustion, the engine is described as "i-DSI," which stands for "Dual & Sequential Ignition." There are two diagonally opposed spark plugs per cylinder. Honda has developed and patented its Twin Plug Sequential Control system that is programmed to respond to engine rpm and load conditions. It may be that the plug located near the intake port ignites first, after the air/fuel mixture enters the cylinder, then the second plug, located near the exhaust port fires. Or both plugs can fire at once. (There is a 30° angle between the intake and exhaust valves to permit a compact combustion chamber.)
The con rods are high-strength forged steel that's treated with a carbon hardening process. This permits the rods to be lighter than might otherwise be the case. The engine has a single overhead cam with a compact chain drive and VTEC (Variable Valve Timing and Lift Electronic Control) system. A single rocker shaft is used for both intake and exhaust, thereby eliminating the need for a second shaft, which helps contribute to reduced weight.
The electric motor/generator is attached directly to the crankshaft. The power for the motor is primarily generated by regenerative braking: as the vehicle decelerates during coasting or braking, the motor becomes a generator, which produces electricity that's stored in the NiMH battery pack* located behind the rear seat. In a conventional engine during deceleration, there's what's known as "engine braking," created by the pumping action of the cylinders. Because engine braking would decrease the efficiency of the generator, there is what is known as the VTEC Cylinder Idling System, which closes the intake and exhaust valves on as many as three of the four cylinders, thereby permitting the pistons to move more freely in the cylinders, reducing engine drag for improved generator efficiency.
So, put together, the system works like this: During acceleration, the electric motor kicks in (with a manual transmission, it provides 46 ft-lb of torque @ 1,000 rpm; with a CVT it's 36 ft-lb @ 1,000 rpm). Otherwise, the vehicle is powered by the internal combustion engine. With the manual, the EPA city/highway numbers are 46/51; they're 48/47 with the CVT.
*The battery box for the Civic Hybrid measures 495 mm x 372 mm x 174 mm, which is 30% smaller than the one used in the Insight. It contains 120 1.2-volt nickel-metal hydride (NiMH) cells. The battery box is contained within what is known as the Intelligent Power Unit, which also includes the Power Control Unit, Electric Control Unit, and a cooling system. Overall, the IPU is 42% smaller than the one in the Insight.