2018 Honda Accord Hybrid. Click to enlarge.

As before, the Accord two-motor system operates without the need for a conventional automatic transmission.

Detailed energy management for each component, as controlled by the Power Control Unit (PCU), enables this system to make the best use of both the electric propulsion motor and the gasoline engine to provide seamlessly smooth and responsive performance while garnering top-class EPA fuel economy ratings in the midsize hybrid sedan class. (EPA rating still pending.)

To help maximize efficiency on the road, the Accord Hybrid seamlessly shifts between three distinct drive operations: EV Drive (100% electric motor); Hybrid Drive (electric motor and gasoline engine [driving the generator motor]); and Engine Drive (gasoline engine), utilizing power from the Atkinson-cycle i-VTEC 4-cylinder engine and the electric motors to suit the driving conditions moment to moment.

• EV Drive Operation. All-electric drive EV Drive operation occurs when starting from a stop, during light cruising and acceleration, and when braking. The gasoline engine is off when the i-MMD is in EV Drive mode and is decoupled from the drivetrain to reduce friction.

• Hybrid Drive Operation. The electric propulsion motor alone powers the front wheels, as the gasoline engine (decoupled from the drive wheels) powers the electric-generator motor, which in turn provides power to the battery pack. This allows the gas engine-generator motor combo to supplement the battery by providing added electrical power to the propulsion motor or, alternatively, to charge the battery. In this type of operation, the system works as a series hybrid. The electric propulsion motor offers crisp acceleration, and can reach maximum torque almost instantaneously, thanks to electrical power that can be supplied from both the gas engine-powered motor/generator and the battery.

• Engine Drive Operation. When cruising at medium to high speeds, the high-efficiency Atkinson-cycle i-VTEC gasoline engine provides propulsion via a high capacity lock-up clutch, which connects the generator motor (always linked to the engine) and the electric drive motor, effectively sending motive power directly from the engine to the drive wheels. In this type mode, the system works as a parallel hybrid in which both the gasoline engine and, when required—for example, upon quick acceleration—the electric propulsion motor, both provide power to the wheels. The i-MMD powertrain operates without the use of a conventional transmission.

2.0-liter engine. The 2018 Accord Hybrid’s third-generation i-MMD system utilizes a 2.0-liter DOHC i-VTEC 4-cylinder Atkinson-cycle engine that develops a peak 143 horsepower at 6200 rpm and 129 lb.-ft. of torque (175 N·m) at 3500 rpm.

Advances in engine control tuning together with other technologies combine to create excellent power characteristics, exceptional fuel efficiency and very low exhaust emissions. Key contributors to the improved thermal of efficient of the engine include improved exhaust gas recirculation and reductions in operating friction. This clean-running engine meets LEV3-AT-PZEV (SULEV30)/Interim Tier3 Bin30 emission standards, while contributing to the Accord Hybrid’s total system output of 212 horsepower.

Since the gasoline engine can be decoupled from the rest of the hybrid powertrain, it only operates as needed. Depending on the state of battery charge and other parameters, the engine will automatically shut off during deceleration and when the vehicle is at a stop or operating under battery power. When needed, the gasoline engine restarts automatically, without action from the driver.

The engine has a displacement of 1,993cc. Cast-in iron cylinder liners provide long-lasting durability. Each journal on the forged-steel crankshaft is micro-polished to reduce internal friction. To improve smoothness throughout the rpm range and help lower noise levels, the Accord Hybrid is fitted with an internal balancer unit. Consisting of a pair of chain-driven counter-rotating shafts located in the oil pan, the balancing system helps quell the inherent second-order harmonic vibrations that are normally associated with inline 4-cylinder engines.

The 2.0-liter engine has cylinder bores offset by 6.0 mm from the crankshaft to help reduce piston-sliding friction. This gives the connecting rods a more favorable angle during each power stroke, which reduces side loading on the pistons and, in turn, improves efficiency. The lightweight pistons have a carefully optimized skirt design to minimize reciprocating weight, and reduced weight minimizes vibration and increases operating efficiency.

The Accord Hybrid engine has a lightweight cylinder head that is made of pressure-cast aluminum alloy. A silent chain drives dual overhead cams and four valves per cylinder. The cam drive is maintenance free throughout the life of the engine.

To help boost fuel efficiency and power, the engine has a 34-degree included angle between the intake and exhaust valves. The narrow valve angle decreases the surface-to-volume ratio and helps create a flatter, more compact combustion chamber that reduces unburned hydrocarbon emissions. This combustion chamber shape and the precise control enables a high compression ratio of 13.5:1, yet the engine still operates on regular unleaded fuel.

The Accord Hybrid’s 2.0-liter DOHC 16-valve i-VTEC engine uses an advanced valve-control system to combine high power output with high fuel efficiency and low emissions. The system combines E-VTC (Electronic Variable Timing Control), which continuously adjusts the intake camshaft phase, with Variable Valve Timing and Lift Electronic Control (VTEC), which changes the lift profile, timing and duration of the intake valves’ operation.

A key contributor to the Accord Hybrid’s excellent emission performance is a single dual-bed catalytic converter. The converter mounts directly to the front of the cylinder head, close coupled for fast activation after the engine is started. It uses a thin-wall design that increases internal reaction area and improves efficiency. A high-efficiency exhaust system and the dual-bed, high-density catalytic converter help the Accord engine meet stringent LEV3-AT-PZEV(SULEV30)/Interim Tier3 Bin30 standards, which require reduced greenhouse gases compared to lower standards.

Heat Recovery Device. The heat recovery device contributes to the third-generation i-MMD's efficiency by helping to reduce power-robbing friction, particularly during cold-weather startup and initial operation, prior to engine warmup. Located downstream on the exhaust system, the heat recovery device uses hot exhaust gas to warm engine coolant quicker, especially during cold weather conditions. This brings the engine up to normal operating temperature quicker, reducing internal friction and allowing the i-MMD to operate in electric mode earlier. This helps improve fuel efficiency during cold weather operation.

Electric Motors. The 2018 Accord Hybrid uses two new-generation electric motors, a propulsion motor that drives the wheels and a generator motor that produces electricity, both of which have been upgraded from the previous generation i-MMD. The Honda-developed electric motors that are the first permanent magnet drive motors in the world to use magnets containing no heavy rare-earth metals. The propulsion motor is rated at 181 horsepower and 232 lb.-ft. (315 N·m) of torque.

When in regenerative mode, the propulsion motor converts the kinetic energy of the decelerating vehicle into electrical energy to recharge the battery. Its operation is seamlessly coordinated with the Accord Hybrid’s electric-servo braking system. A separate generator motor is driven by the gasoline engine to generate electric energy to drive the propulsion motor when the vehicle is operating in Hybrid Drive Mode. This generator motor can also restart the gasoline engine when the vehicle is in idle-stop mode.

Intelligent Power Unit (IPU). The Accord Hybrid’s completely reworked intelligent Power unit (IPU) is now 32% more compact and has been repositioned under the rear seat base, instead of in the trunk, behind the rear seatback as in the previous generation Accord Hybrid. This repositioning means that the Accord Hybrid offers the same roomy trunk utility and 60/40 split-folding rear seatback versatility of the non-hybrid Accord models. The IPU packages a reconfigured Li-Ion battery pack that powers the Accord Hybrid’s electric propulsion motor. Improved battery control systems and revised battery chemistry allow the new battery to operate over a broader state-of-charge range for improved efficiency.

The battery pack is recharged by the propulsion motor operating in regenerative braking mode, and by the generator motor, which is powered by the gasoline engine. Battery temperature is controlled by a fan system that pulls air from the interior of the vehicle via vents located beneath the rear seat bottom. The battery pack is covered by a special warranty that covers defects in material and workmanship for eight-years/100,000-miles or ten-year/150,000 miles depending on the state of purchase/registration.

Power Control Unit (PCU). The Power Control Unit (PCU), which dictates the power management strategy of the two-motor hybrid system, including motor power and battery recharge, now includes the DC/DC converter and is mounted in the engine compartment.

Drive Force Transfer. Just like the previous model, the Accord Hybrid is not equipped with a conventional mechanical transmission. Instead, motive force transfer is accomplished through the interaction of the Accord Hybrid’s gasoline engine and two electric motors. Coordinated by the IPU, this form of drive force transfer offers smooth and predictable acceleration matched with efficient low-rpm highway cruising when the gasoline engine is in operation. Gasoline engine shutdown is seamlessly integrated into the operation of the Accord Hybrid when appropriate.

The drive force transfer system operates without the need for a torque converter, mechanical pulley or belt. It instead uses two motors for driving and generating power. The system is optimally and rapidly able to control both engine and electric motor rotation in order to deliver higher fuel efficiency and quicker engine response in each driving mode.

When cruising at mid- or high-speeds in the high-efficiency range of the engine, a lock-up clutch is engaged, connecting the drive motor to the generator motor to transmit engine torque directly to the drive wheels as efficiently as possible. In EV Drive operation, when the battery-powered drive motor is used for either acceleration or regenerative braking, a clutch disengages the stopped gasoline engine from the drivetrain to eliminate efficiency loss from mechanical friction in the engine.

Improving efficiency and reducing weight and size, the drive force transfer system of the second-generation i-MMD integrates the torque limiter within the flywheel.

Deceleration Selector Paddles. In the Accord Hybrid, new steering wheel-mounted selector paddles allow the driver to select between four levels of regenerative braking performance. The right paddle adds regenerative braking (displayed visually on the instrument panel) and the left paddle reduces regenerative braking.

Common features for all engines. The Accord’s Drive-by-Wire throttle system replaces a conventional throttle cable with smart electronics that “connect” the accelerator pedal to a throttle valve inside the throttle body. The result is less under-hood clutter and lower weight, as well as quicker and more accurate throttle actuation. Moreover, a specially programmed gain rate between the throttle pedal and engine offers improved drivability and optimized engine response to suit specific driving conditions.

Honda’s Drive-by-Wire throttle system evaluates the current driving conditions by monitoring throttle pedal position, throttle valve position, engine speed (rpm) and road speed. This information is used to define the throttle control sensitivity that gives the Accord’s throttle pedal a predictable and responsive feel that meets driver expectations.

All Accord engines have their exhaust manifold cast directly into the aluminum alloy cylinder head to reduce weight and complexity, and to position each primary catalytic converter as close as possible to the combustion chambers. A high-efficiency converter mounts directly to the exhaust port of the cylinder head for extremely rapid converter activation after engine startup.

All the Accord’s engines also make use of friction-reducing technologies designed to improve engine efficiency. The outer skirts of lightweight aluminum pistons feature a low-friction coating applied in a unique pattern. The result is reduced overall friction as the pistons move within the cylinder bores.

Plateau honing further lowers the friction level between the pistons and the cylinders by creating an ultra-smooth surface. Plateau honing is a 2-stage machining process that uses two grinding processes instead of the more conventional single honing process. This also enhances the long-term wear characteristics of the engine. Low viscosity oil (0W-20) also reduces friction. Other contributors to overall operating efficiency are a special 2-stage oil pump relief valve, low-friction oil seals, special low-drag piston ring design, low-friction cam chain and a lightweight crankshaft.