642 Recent Addition To Production Hybrid Vehicles

Honda's Insight hybrid-drive car, Fig. 6.16, uses the company's Integrated Motor Assist (IMA) hybrid system, comprising high efficiency petrol engine, electric motor and lightweight 5-speed manual transmission, in combination with a lightweight and aerodynamic aluminium body, seen at (a), to provide acceleration of 0 to 62 mph in 12 seconds and a top speed of 112 mph, without compromising fuel economy of 83 mpg (3.41/100 km) and 80 g/km CO2 (EUDC) emission. The car is claimed to have the world's lightest 1.0 litre, 3-cylinder petrol engine, which uses lean burn technology, low friction characteristics and lightweight materials in combination with a new lean burn compatible NO catalyst.

The electrical drive consists of an ultra-thin (60 mm) brushless motor, (b), directly connected to the crankshaft, (c), 144 V nickel-metal hydride (Ni-MH) batteries (weighing just 20 kg) and an electronic Power Control Unit (PCU). The electric motor draws power from the batteries during acceleration (so-called motor assist) to boost engine performance to the level of a 1.5 litre petrol engine as well as acting as a generator during deceleration to recharge the batteries. As a result engine output is increased from a high 50 to 56 kW with motor assist, but it is low speed torque that mainly benefits, boosting a non-assist 91 Nm at 4800 rpm to 113 Nm at 1500 rpm.

A new type of lightweight aluminium body, (d), offers a high level of rigidity and advanced safety performance. It is a combination of extruded, stamped and die cast aluminium components and body weight is said to be 40% less than a comparable steel body. All outer panels are aluminium except for the front wings and rear wheel skirt which are made from recyclable abs/nylon composite. Total kerb weight is 835 kg (850 kg including air conditioning). Aerodynamic characteristics include a streamlined nose, a low height and long tapered roof, narrow rear track, low drag grille, aluminium aero wheels, rear wheel skirt, a flat underside, and a tail designed to reduce the area of air separation. Insight also uses low rolling resistance tyres that have been designed to provide good handling, ride comfort and road noise characteristics. All these features give the Insight an aerodynamic drag coefficient of 0.25.

Further fuel savings are provided by an auto idle stop system. In simple terms, the engine cuts out as the car is brought to a standstill, and restarting is achieved by dipping the clutch and placing the car in gear. In combination, Honda calculates that weight reduction measures, aerodynamics and reduction of rolling resistance contribute to approximately 35% of the increase in fuel efficiency, and the IMA system a further 65% compared to a 1.5 litre Civic. Further features include ABS, electric power steering, dual air bags, AM/FM stereo cassette, power windows and mirrors, power door locks with keyless entry, automatic air conditioning and an anti-theft immobilizer.

Fig. 6.16 Honda Insight hybrid: (a) aerodynamic tailed body and underbonnet power unit; (b) motor; (c) motor installation; (d) body structure.

The battery system is designed to avoid overcharging or complete emptying and in the unlikely event of motor failure, the Insight will run on the petrol engine alone. At the front, the suspension consists of struts, with an aluminium forged knuckle and lower arm, anti-roll bar linked to the dampers, and light aluminium cast wheels; while at the rear, a light and compact suspension features a twist beam with variable cross-section, and trailing arms with bushes having a toe-control function. Electric power steering, optimized for feel and feedback, has been used to make further fuel savings. It features a centre takeoff and aluminium forged tie-rod.

The company argue that in conventional petrol/electric hybrid systems, the vehicle is powered by the electric motor alone at low speeds. At higher vehicle speeds, or when recharging is required, engine torque is directed to the driven wheels or used to drive a generator. Such systems require complex control mechanisms, large capacity batteries, as well as a separate motor and generator. Honda chose instead a system in which the motor is linked directly to the engine, assisting it during acceleration for a reduction in consumption and acting as a generator during deceleration. When cruising, there is no assistance and lean burn keeps fuel consumption to a minimum. A very wide, flat torque curve is achieved through the benefits of VTEC technology at high engine speeds and the substantial boost provided by the electric motor at low and mid-range engine speeds. This approach allows for superior fuel efficiency and excellent driving performance over a wide range of driving situations.

The key to the engine operating at exceptionally low air/fuel ratios is rapid combustion of the mixture, since combustion time increases as the mixture becomes leaner. By adopting a new swirl port to enhance the turbulence of the mixture in the cylinder, a compact combustion chamber and a high compression ratio are achieved. The design is an evolution of the conventional VTEC-E mechanism where swirls are generated by almost closing one of the pair of inlet valves. In the new design, the inlet ports are set up in a more vertical direction to generate more powerful swirls flowing into the cylinder. This has been made possible with a new VTEC mechanism. Rather than inlet and exhaust rocker arms carried on separate rocker shafts, the Insight features just one rocker shaft with the included angle of the valves narrowed from 46 to 30°, allowing the high swirl port shape and the compact combustion chamber to be realized. Conventional lean burn engines, with their oxygen rich exhaust gases, mean reducing NO emissions is technologically difficult. The Insight's improvement in combustion efficiency goes some way towards solving the problem. However, a newly developed catalytic converter containing additives able to absorb NO , provides an elegant solution to the problem. During lean-burn driving, NO is directly absorbed; it is later reduced to harmless nitrogen in stoichiometric driving conditions. The system also helps to boost fuel efficiency, since it allows a widening of the lean burn range and therefore improved efficiency. Emissions performance is further improved by an exhaust manifold-integrated cylinder head. Rather than a conventional arrangement of an independent exhaust port for each cylinder, the ports are combined into one in the cylinder head structure. Considerable weight reduction is the result, but just as important, the small radiation area minimizes heat loss, enabling quick activation of the catalytic converter.

New technologies have reduced the overall friction of the engine by 38% compared with a conventional 1.5 litre engine. Among the measures adopted are roller type rocker arms, adapted to the single cam VTEC mechanism, providing a 70% reduction in friction losses. A special 'micro dimple' surface treatment of the piston skirt improves the retention of the oil film between the piston and the cylinder reducing friction by approximately 30%, in conjunction with offset cylinders and low tensile piston rings. By using case hardening for significantly increased strength, slimmer connecting rods have been adopted, achieving a reduction in weight of 30%. A newly developed magnesium alloy, with a high degree of heat resistance, has been used for the engine sump in place of aluminium alloy, giving a 35% weight reduction. Other weight saving technology includes: a thin sleeve block, the new VTEC cylinder head, bracketless ancillary equipment, a magnesium PCU case, and an increase in plastic parts (intake manifold, cylinder head cover, water pump pulley).

The ultra-thin brushless motor of10 kW output sandwiched between the engine and transmission has a central rotor manufactured using the lost wax method, to give a precise shape and high strength, which achieves a 20% weight reduction. For the rotor magnet, improvements to the neodymium sintered magnet used in the Honda EV Plus mean an improvement in the magnetic flux density or torque ratio by 8%, while improved heat resistance has made a cooling system unnecessary. In order to create a thin motor, a split stator with compact salient-pole field winding and centralized bus ring forms a very simple structure allowing a width of 60 mm, 40% thinner than if conventional technologies were used. The Ni-MH battery pack installed at the rear of the car is held in a compact cylindrical pack. A series connection of 120 cells each with 1.2 V provides a voltage of 144 V. Ni-MH batteries are said by the company to offer stable output characteristics regardless of the charging condition, as well as excellent durability. The Power Control Unit (PCU), mounted alongside the battery pack, provides precision control of the motor assist and battery regeneration functions, as well as the supply of electricity to the standard 12 V battery through a DC/DC converter. The inverter which drives the motor, and is the most important element in the PCU, consists of a compact 3 phase integrated type switching module.

The weight target was a body-in-white of 150 kg, or half that of the Civic 3 door, the closest comparable sized Honda model; compared with the Civic it was reduced to 47%, yet torsional rigidity is up by 38%, and bending rigidity up by 13%. Hexagonal extruded aluminium cross-sections are used for the front side frame, bringing a weight saving of 37% while also attaining high energy absorbing characteristics compared to a conventional steel frame. The side sill and roof side rail which contribute considerably to the overall body rigidity, although simpler in cross-section, achieve 47% and 53% weight reductions respectively. A new manufacturing technique, 'three-dimensional bending forming', provides a degree of freedom in design, and a reduction in the number of parts required has been adopted, for example to produce the roof side rails. Widely different sections and the need for high rigidity called for the special jointing method involving die cast aluminium, which permits a high degree of shaping and flexibility in joining different shaped sections. However, in the case of the rear outrigger, where structural frames meet from three directions and which serves as the installation area for the suspension frame, an alternative was required. Its deep box-like shape means that if it were formed with the conventional die cast method its wall thickness would become too thick and too heavy. So the thixo-cast method was used, said to be a first in body frame construction. This involves pouring aluminium in a half solidified rather than molten state to create a uniform and fine metal structure allowing a 22% thinner wall thickness, 20% higher strength, and a 20% weight reduction. In comparison with the NSX, the Insight uses 15% fewer body parts and 24% fewer welding spots to give weight and productivity savings.

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