Honda Accord -- ULEV Engine and Transmission

9/15/2000 9:27:43 PM


The ULEV (Ultra-Low Emission) engine used on the Accord EX model with automatic transmission uses several new technologies to achieve its remarkably low emissions output. In addition to the electronic EGR control and lean combustion afforded by the VTEC swirl-combustion process, the ULEV engine uses precise air-fuel control made possible by a Linear Air-Fuel sensor, secondary oxygen sensor, high-performance catalyst and new 32-bit fuel-injection microprocessor.

A special low heat-mass exhaust system and underfloor catalyst aid in achieving a lean air-fuel mixture during the critical warm-up period of engine operation.

The ULEV engine-programmed fuel-injection system is an adaptive type that uses a new 32-bit microprocessor Powertrain Control Module (PCM) with the capability to monitor and control the air-fuel ratio in each individual cylinder. In addition to the normal sensor inputs, such as throttle position, coolant temperature, air temperature, etc., the ULEV engine has two air-fuel ratio sensors: a linear air-fuel sensor in the exhaust manifold and a stoichiometric sensor between the front and rear beds of the underfloor catalytic converter.

The ULEV engine PCM can also compensate for mixture inaccuracies caused by fuel-droplet adhesion to the intake port walls during cold start-up (a major reason for a high level of unburned hydrocarbons during the warm-up cycle).

The ULEV fuel-injection system also utilizes the precise EGR control provided by the Accord engine's electronic EGR valve.

The ULEV engine uses a rotary air-control valve that also controls the Air-Assist Injection system, in place of the electric air-control valve and fast-idle valve used in the previous Accord.

The exhaust system solves the problem of passing combustion heat to the catalyst as soon as possible. Since a large mass of metal, such as a cast manifold, can act as a heat sink, the manifold is a dual-wall design made out of tubular steel. An air gap between the manifold's inner and outer walls insulates the system and ensures that exhaust heat continues to the catalyst. The manifold is a four-into-one branch design.

The ULEV Accord engine uses a 1.7-liter, high-efficiency catalyst. The catalyst is located under the floor and uses a substrate with a higher Palladium content and modified precious-metal layering. The cell number was increased to 600 cells per square inch for faster lightoff, while providing more surface area to convert hydrocarbons.


The Accord V-6 and 4-cylinder models benefit from a direct-control automatic transmission. The transmission shifts precisely with reduced shift shock, is compact and weighs less than the model it replaces. The transmission is produced in Honda's HTM (Honda Transmission Manufacturing) plant in Ohio.

The Accord automatic is a constant-mesh type, with three parallel shafts: input, intermediate and output. Shifting is electronically controlled and hydraulically actuated. Shift management is controlled by a 16-bit microprocessor. The new microprocessor's greater capacity is used to control two new linear-shift solenoids, that in turn control hydraulic pressure to the gear clutch packs.

A centrifugal hydraulic cancellation mechanism allows precise shift control by minimizing centrifugal hydraulic pressure in the clutch packs.

The 16-bit powertrain control module also receives throttle-position data electronically, making it possible to eliminate the throttle-cable-actuated hydraulic control used on previous Accord transmissions and resulting in lighter throttle pedal operation.

The 4-cylinder Accord transmission fits in a 22-mm shorter case than its predecessor, while the 6-cylinder transmission was lightened by some 2.3 kg, despite retaining the same external dimensions.

Gear noise reduction was achieved through a variety of improvements. The intermediate and output shafts are now held in place by three support bearings. This reduces shaft bending and maintains proper mesh contact even under load. Similarly, the gear-tooth mesh contact ratio has increased, which also further reduces gear noise.

The lockup torque converter helps minimize fluid-coupling slippage by mechanically coupling the engine to the transmission during certain driving modes, such as steady-state cruising. The benefit is better mileage. The lockup feature works in 3rd and 4th gears, and also maintains lockup in 3rd and 4th gears during deceleration. The design used in the Accord utilizes a longer torsion damper spring around the outer circumference of the lockup-clutch disc. The design absorbs vibration and judder better during engagement and allows for an extended lockup zone. The result is a smoother engagement and also an increase in efficiency, and therefore higher mileage.

Cruise control is governed by the auto-cruise control ECU in combination with the Powertrain Control Module, which operates the automatic-transmission and fuel-injection systems. The PCM determines when vehicle speed drops below the speed set by the cruise control and initiates the appropriate gear selection, if necessary.

The Accord 4-speed automatic transmission uses Honda's Grade Logic Control System. Grade Logic differs from more conventional computer-controlled shift programming in two important ways: It can determine certain vehicle driving situations and then, based on stored "shift maps," select the appropriate shift points for 3rd-gear/4th-gear upshifts and 4th-gear/3rd-gear downshifts. Four-cylinder engine Accords, equipped with the automatic transmission, have a 2nd-gear/3rd gear, 3rd-gear/4th-gear upshift, and a 4th-gear/3rd-gear, 3rd-gear/2nd-gear downshift sequence. This feature is especially useful when driving up and down long grades and when performance-driving.

A cable-operated, 5-speed manual transmission is standard on the Accord. Double-cone synchronizers on 2nd gear help minimize shift effort. A smaller diameter clutch lining further reduces shift efforts while maintaining durability.