Broadly, Honda estimates that fuel economy of the vehicle, compared to a hypothetical conventional gasoline-engined car with a four-stroke engine (minus features such as cooled EGR) is enhanced approximately 39% via the basic hybridization using the two-motor system. The fuel consumption (BSFC) of the engine is improved approximately 10% using a combination of cooled EGR and VTEC. Efficiency enhancements of the motor and generator contribute another 10%.

Use of the electric servo brake system realizes another 8% enhancement relative to a conventional regeneration brake system, and low rolling resistance tires and decrease in aerodynamic drag contribute another 3%, for an approximate total 70% improvement in fuel economy compared to a conventional vehicle.

New 2.0L gasoline engine. Honda developed the 2.0L gasoline engine for the Accord plug-in hybrid as a next-generation Honda engine series. The new engine offers a 10% improvement in fuel consumption compared to the current 2.0L engine, and delivers maximum power of 105 kW at 6200 rpm and maximum torque of 165 N·m (122 lb-ft).

The Atkinson cycle engine features Honda’s variable valve timing and electric control (VTEC) and cooled EGR; the basic concept of the engine is to improve fuel consumption by introducing cooled EGR gas in the high load domain. That, however, decreases combustion stability, and required the development of a new control system.

With the VTEC system, each cylinder has three cams and three rocker arms; the end cams are the output cams, while the center cam is the fuel economy (FE) cam. When VTEC is off, the three rocker arms operate separately. When VTEC is on, the three rocker arms are combined by synchronization pins. The output cams specialize in output and emissions performance; the FE cam specializes in fuel economy.

Because the use of cooled EGR to improve fuel consumption slows combustion speed, Honda needed to improve combustion in order to maximize the effect of cooled EGR. Honda engineers designed a high tumble inlet port. The tumble ratio of the developed port is 1.40, while that of the current port is 0.73. Honda engineers reported a 5 g/kWh improvement in indicated specific fuel consumption (ISFC) with the new port.

At high load, Honda advances IVC and increases EGR to a limit of combustion (approximately 16-18% EGR rate) and delivers its best brake specific fuel consumption (BSFC) (about 216 g/kWh).

At middle loads, there is a balance point between pumping loss reduction by IVC retard and EGR rate. By adjusting IVC and the EGR rate according to engine load, Honda improves fuel consumption in both high and middle loads ranges. Having the FE cam makes this possible.

EGR control. Gasoline engine EGR has been conventionally used to reduce pumping loss and emissions, and was adopted mainly in the low- to mid-load domains. The new Honda engine, however, introduces EGR at high loads, which makes the control of the flow rate more significant.

Generally, as the pressure difference is closer to 0, the sensitivity of the valve flow rate increased drastically. As a result, flow rate control by valve operation becomes difficult. To obtain accuracy of EGR flow rate, we constructed new control to keep the specified differential pressure regardless of environmental change.

—Yonekawa et al.

Torque control. With the high compression ratio of the engine (13.0), Honda decreases torque by retarding ignition timing to avoid knocking. Decreasing torque, however, results in worse driveability and fuel consumption, so Honda needed to estimate the torque decrease correctly. To do so, Honda engineers developed a new torque estimation model. Various kind of operating conditions can thus be controlled to compensate for the reduction in torque to maintain driveability and fuel consumption.

Operating point control. Honda found that decreasing atmospheric pressure worsened fuel consumption, as EGR is reduced; the optimal fuel consumption line moves with atmospheric pressure change. Honda therefore developed new controls to adjust the operating line according to atmospheric pressure. Engineers established two bounding operating lines for normal condition and an ultimate high-altitude condition. The optimal operating line is established by interpolating the two prepared lines with the measured pressure.

SULEV20. Honda’s development target was to meet the new California LEV3/SULEV20 emissions standard. (Earlier post.) Honda developed a new catalyst warm-up control that made it possible efficiently to warm-up the emissions catalyst for the hybrids.

At engine start, engine load is kept low, reducing the amount of feed emission. After a close-coupled (CC) catalyst warms up, Honda increases the engine load. Exhaust gas energy is increased, which accelerates the warm-up of the under-floor catalyst. The exhaust feed to the under-floor catalyst has already been purified by the CC catalyst. This system keeps the NMOG and NO_xdischarge under the SULEV20 target.

Two-motor plug-in hybrid system. In addition to the 2.0L Atkinson engine, the plug-in system includes an electric-coupled CVT (e-CVT), integrated Power Control Unit (PCU), and Intelligent Power Unit (IPU) including the battery pack, cooling fans, DC/DC converter, on-board charger and battery control unit. The CVT is mated with the engine in the position of a conventional transmission. The PCU is positioned above the e-CVT and cooled by an independent water cooling circuit. Functions of the PCU include high-voltage inverters, Voltage Control Unit (VCU) and motor/generator ECU (Electric Control Unit).

The system can be operated in three modes:

• EV drive mode, in which the electric motor physically connected to the driveshaft propels the vehicle by using energy stored in the battery pack;

• Hybrid mode, in which the electric motor is driven by electric energy generated by the generator using output form the engine; and

• Engine drive mode, in which the engine-drive clutch located between the engine and the wheels is engaged, and the vehicle is propelled directly by the engine output.

The e-CVT contains the traction motor and a generator motor and has a three shaft configuration (input, counter, and output shafts). It also has two mechanical oil pumps to support the three driving modes. One, mounted on the inner shaft of the e-CVT, controls the engine-drive clutch and cools the motor and generator. The other is for lubrication, and is mounted on the output shaft.

The synchronous 124 kW motor delivers maximum torque of 307 N·m (226 lb-ft) and has a maximum speed of 12,584 rpm. The permanent magnet motor utilizes reluctance torque, high-speed rotation and voltage amplification up to 700V to meet power driving requirements.

During low load operation (e.g., city driving or highway cruising), Honda seeks to reduce magnet torque and increase reluctance torque. To maximize reluctance torque, Honda developed a distributed winding motor with a rotor using a V configuration for its magnets. Motor torque is enhanced 82% by utilizing reluctance torque, Honda said. To achieve high speed rotation, Honda made slit structures on the rotor yoke.

The battery voltage is amplified to achieve both miniaturization and higher power; output power is boosted more than 50% due to the voltage amplification up to 700V.

The 6.7 kWh battery pack is built with 20.8 Ah cells with a voltage of 3.2V. To decrease the weight of the pack, Honda is using frames and covers made of aluminum and plates of magnesium. Cell voltage sensors are installed in both sides of the pack, and monitor every cell for equalization.

The on-board air-cooled 6.6 kW charger is based on SAE J1772.

Resources

• Yonekawa, A., Ueno, M., Watanabe, O., and Ishikawa, N., “Development of New Gasoline Engine for ACCORD Plug-in Hybrid,” SAE Technical Paper 2013-01-1738, 2013, doi: 10.4271/2013-01-1738

• Higuchi, N., Sunaga, Y., Tanaka, M., and Shimada, H., "Development of a New Two-Motor Plug-In Hybrid System,” SAE Technical Paper 2013-01-1476, 2013, doi: 10.4271/2013-01-1476