Honda S2000 -- Powertrain

— Torrance, Calif.

POWERTRAIN

OVERVIEW
The S2000 powertrain uses a front-engine/ rear-wheel-drive layout. The longitudinally mounted engine is mated to a 6-speed, close-ratio manual transmission. A propeller shaft carries the output of the transmission to a frame-mounted limited-slip differential. The differential drives the rear wheels via separate axle shafts.

The 2.0-liter, aluminum-alloy, inline 4-cylinder S2000 engine produces 240 hp @ 8300 rpm and 153 lb.-ft. of torque @ 7500 rpm. The cylinder head is also made of aluminum alloy and features dual overhead camshafts, 4 valves per cylinder and VTEC (Honda's variable valve-timing system) on both the intake and exhaust valves. Fuel induction is via Honda's sequential, Multi-Point Programmed Fuel Injection (PGM-FI) and the ignition system is a high-voltage, computer-controlled direct type with individual spark coils for each cylinder.

The S2000 engine is 9 percent smaller and 10 percent lighter than a 2.2-liter Prelude engine, and almost as small as a 1.6-liter Civic engine. Yet this compact, lightweight engine (326 lbs.) has the highest specific power output (120 hp per liter) of any normally aspirated 2.0-liter production engine in the world. In addi-tion, the engine's exhaust emissions are so low that the S2000 qualifies as a Low-Emission Vehicle (LEV).

The following are the main features of the S2000 drivetrain:

2.0-Liter VTEC Engine

  • 9000 rpm rev limit
  • 240 hp @ 8300 rpm
  • 153 lb.-ft. of torque @ 7500 rpm
  • 0-60 mph acceleration in less than 6 seconds
  • Compact, lightweight engine design aids in ideal (50/50) weight distribution
  • FRM (Fiber-Reinforced Metal) cylinder liners
  • 87 mm bore X 84 mm stroke contributes to high-rpm operation
  • Rigid aluminum (ladder-type) main bearing support, with cast-iron bearing inserts, enhances engine durability
  • Lightweight forged-aluminum pistons and heat-treated (carburized), forged-steel connecting rods add durability
  • Full-floating piston pins eliminate piston noise during warm-up
  • High-volume oil pump with silent chain drive
  • Compact, high-efficiency oil cooler
  • Cast-aluminum oil pan helps minimize engine noise and enhances oil cooling
  • 11:1 compression ratio
  • Compact, DOHC VTEC cylinder head and valvetrain
  • Lightweight, MIM (Metal-Injection Molded), sintered-steel rocker arms
  • Low-friction roller-bearing cam-followers
  • Compact, 2-stage cam-drive with silent chain, scissors gears and fully automatic tensioner
  • Hollow camshafts function as lubrication path for VTEC valvetrain
  • Sequential, Multi-Point Programmed Fuel Injection (PGM-FI)
  • Intake system with straight inlet ports and low-back pressure exhaust system
  • Electric-motor-driven, multi-port secondary air-assist injection system helps lower hydrocarbon emissions
  • Meets California's stringent Low-Emission Vehicle (LEV) standard
  • Compact engine ancillary drive system uses a serpentine drive belt with automatic tensioner
  • Compact engine ancillaries, such as air-conditioning compressor and water pump, save weight and take up less space
  • Direct-ignition system with long-lasting platinum-tipped spark plugs
  • Low back pressure, metal-honeycomb catalytic converter2

Transmission

  • Longitudinally mounted 6-speed, close-ratio manual transmission
  • Transmission lubrication pump enhances durability
  • Reduced shift effort from double-cone synchronizers on first, third and fourth gears, and triple-cone synchronizers on second gear
  • Short-throw, direct shift linkage
  • Compact pull-type clutch mechanism and high-performance clutch
  • Low-mass flywheel
  • Propeller shaft uses constant-velocity joints instead of U-joints
  • Torsen limited-slip differential
  • Highly rigid axle shafts utilize stronger flanged ends in place of splines

S200 ENGINE DESIGN2
Honda engineers designed the S2000 with an ideal 50/50 weight distribution, a low yaw moment and a low center of gravity. They accomplished this by placing the engine and drivetrain as far back in the chassis as possible, and by designing the engine to be very compact. In addition, many of the engine's components have been designed to be as compact and as lightweight as possible.

COMPACT HIGH-OUTPUT ENGINE BLOCK
The S2000 engine block is a one-piece, open-deck aluminum-alloy die casting. Of special note are the block's FRM (Fiber-Reinforced Metal) cylinder liners cast integral with the block. FRM is a composite material consisting of carbon fibers embedded in an aluminum oxide matrix (aluminum oxide is a ceramic material used for spark-plug insulators). As a cylinder lining, FRM offers several advantages over conventional ferrous-metal liners, including lower weight, faster heat transfer and a greater resistance to wear.

Cylinder bore is 3.42 in. (87 mm) and the stroke is 3.30 in. (84 mm). This results in a nearly "square" 1: 0.96 bore-stroke ratio that facilitates high-rpm performance and still exhibits good torque characteristics at lower rpm.

FORGED-ALUMINUM PISTONS
The high-revving nature of the S2000 engine necessitated the use of special high-strength, lightweight forged-aluminum pistons (the first use of this material in a Honda production automobile). The piston's minimal skirt area contributes to friction reduction.

The carburized connecting rods and crankshaft, also steel forgings, are heat-treated for added toughness.

LADDER-TYPE MAIN-BEARING CARRIER
The S2000 engine's large ladder-type, cast-aluminum stiffener, with cast-iron bearing inserts, runs the full length and width of the lower engine block and contributes considerably to engine rigidity. The carrier also incorporates oil passages for the main bearings.

CAST-ALUMINUM OIL PAN
A cast-aluminum oil pan bolts to the bottom of the main-bearing carrier. The pan is finned to help dissipate heat, and the use of an aluminum casting instead of a steel stamping provides additional rigidity to the engine and transmission. In addition, cast-aluminum radiates less engine noise than a customary steel-stamped pan.

NEW DOHC VTEC CYLINDER HEAD
The S2000 engine's DOHC cylinder head is a new, highly compact design, and like the engine block, is an aluminum-alloy die casting. The combustion chambers are a pent-roof shape, with four valves for optimum high-rpm airflow. A narrow (51-degree) included angle between the intake and exhaust valves helps to concentrate air and fuel around the central spark plug, resulting in more complete combustion and greater efficiency.

The valve springs are a single-element, round-profile type whose high-rpm design borrows heavily from Honda's racing-engine building experience.

VTEC VARIABLE VAVE TIMING
The S2000 engine uses a performance version of Honda's innovative variable valve-timing system on both the intake and exhaust valves (VTEC stands for Variable Valve-Timing and Lift Electronic Control). VTEC maximizes the S2000 engine's volumetric efficiency -- packing the maximum amount of air and fuel into the combustion chamber on each intake stroke and expelling the maximum amount of exhaust gases on the exhaust stroke.

VTEC works by varying valve timing and lift to compensate for the time delay and out-of-phase arrival of the air-fuel charge at the intake valve. Ideally, the valves should remain open for a short duration at low engine speeds and for a longer duration at high engine speeds -- and that is precisely how VTEC works.

LOW-AND MEDIUM-SPEED OPERATION
In the S2000 VTEC engine, each intake and exhaust valve uses two different cam-lobe profiles: one for low engine speeds and a second for high engine speeds. From idle to around 6000 rpm, the two intake and exhaust valve cam followers at each cylinder are actuated by low-rpm cam lobes. Their short duration and low lift ensures good cylinder-filling at low engine speeds.

HIGH-SPEED OPERATION
At around 6000 rpm (depending on throttle position), an electronic control unit commands a spool valve to open and send oil pressure to pins in the cam followers. Under pressure, the pins lock the two intake-valve followers and the two exhaust-valve followers to a third follower. Until this moment, this third follower has been independently following the contour of a separate high-lift, long-duration cam lobe. Now the valves are actuated by the third follower and more closely match the induction and exhaust timing required for optimum torque at high engine speeds.

NEW COMPACT CAMSHAFT DRIVE
The S2000 engine's dual overhead camshafts feature a new space-efficient cam-drive consisting of a crankshaft-driven, silent-chain primary drive and a geared-secondary drive. The chain, along with a chain guide and an automatic tensioner, is located in an enclosed gallery at the front of the engine block.

The primary chain turns an idler gear at its upper end, which drives the second stage -- the intake and exhaust camshaft gears. The camshaft gears are smaller in diameter than conventional toothed sprockets, which allow the camshafts to be placed closer together, further saving space. Geared drives are widely used in racing engines because of their dependability and greater timing accuracy at high rpm.

Each camshaft gear is a split (scissors) type, consisting of two concentric, spring-loaded gears, set at a slight angle from each other. When engaging the teeth of the idler gear, the spring-loaded split teeth of the cam gear take up any backlash, ensuring smoother, quieter operation.

LIGHTWEIGHT VTEC CAM FOLLOWERS
Because of the high-rpm nature of the S2000 engine and the need to save space, Honda engineers devised a new VIEC cam follower system. The central element in this newest DOHC VTEC system is the development of a new roller-type, coaxial VTEC cam follower. The adoption of a roller in the area in contact with the camshafts helps further reduce friction losses. At the same time, a reduced inertial moment has been made possible by integrating the sliding pin used to operate the cam profile switch into the roller structure.

A new, more precise metal-injection molding process has also been adopted in order to obtain the higher degree of rocker-arm finish required by the roller-type, coaxial VTEC design. These innovations have reduced valvetrain friction by approximately 70 percent, and improved engine torque characteristics by allowing the valvetrain changeover point to be set at a relatively low 5850 rpm. This allows the use of the high-performance, high-lift cam profile over an extended range of engine speeds, so engine response and power output remain high all the way to its 9000 rpm redline.

The two overhead camshafts are hollow in order to supply oil to the cam lobes for lubrication. This eliminates the need for a separate oil line and nozzles, which helps to simplify the cylinder-head lubrication system.

COAXIAL ROLLER-BEARING CAM FOLLOWERS
The cam followers use roller-element bearings to help minimize friction. An additional space efficiency was realized by placing these roller elements so that they are concentric with the hydraulic pistons of the VTEC system.

POWDERED-METAL, INJECTION-MOLDED,SINTERED STEEL-ALLOY CAM FOLLOWERS
Honda engineers used a powdered-metal injection-molding process to make the S2000 engine's VTEC cam followers, in place of the older, more complicated and time-consuming method of casting and machining them. In this new process, powdered steel alloy is mixed with a binder that allows it to be injected into a mold, in much the same way that plastic items are injection-molded. The part is then removed from the mold and the binder is removed by heating. The rocker arm is then sintered, which involves heating the metal to just under its melting point in a special furnace so that the steel particles weld together. Complex shapes such as the S2000 engine's cam followers are more easily produced by metal injection-molding, and they require little additional finishing.

EXTERNALLY MOUNTED OIL PUMP
To help minimize engine length, Honda engineers placed the oil pump on the side of the engine block instead of its usual location at the front of the crank-shaft. This new placement improves engine compactness, and the pump scavenges oil more efficiently. Additionally, the oil pump's lower placement helps pressurize the lubrication system more quickly during engine startup.

The oil pump itself also is a new, more compact design that uses a smaller, high-speed rotor and suction-pickup ports on both sides of the pump body. The new design supplies a greater volume of oil to the engine at all engine speeds. The oil pump is driven via a silent chain connected to the crankshaft.

COMPACT ENGINE ANCILLARY DRIVE
The system for the engine ancillaries, such as the alternator, air-conditioning compressor and water pump, can take up considerable space at the front of the engine. So Honda engineers moved them to a new location at the side of the engine block (a practice commonly used on racing engines) and designed a new, compact drive system that uses both sides of a serpentine belt. An automatic tensioner is built into the drive system.

In addition, many of the engine's ancillary components have been designed to be more compact and easier to manufacture. For example, the water-pump housing incorporates the thermostat and the alternator-mounting bracket into one unit. The oil cooler is a new water-cooled design, and the air-conditioning compressor uses a smaller diameter clutch and armature.

Detail changes to the ancillaries and their drive system on the 2.0-liter S2000 engine allowed Honda engineers to pare about two inches (50 mm) from the front of the engine, compared to Honda's 2.2-liter Prelude engine, making it around the same size as the smaller 1.6-liter Civic engine.

DIRECT-IGNITION SYSTEM
In place of a distributor, the S2000 engine uses a computer-controlled direct-ignition system, with individual high-voltage coils located at each spark plug. The spark plugs have non-fouling platinum tips for longer life.

Timing data for the ignition system is supplied to the engine's Electronic Control Module by a pair of TDC (Top-Dead Center) sensors -- one located on each camshaft -- and a toothed-wheel crank-angle sensor located on the crank-shaft. The Electronic Control Module automatically adjusts engine spark timing and dwell based on throttle opening, engine rpm, knock-sensor data, etc.

Eliminating the distributor saved over six inches (159 mm) from the rear of the engine (compared to the 2.2-liter Prelude engine), which allowed it to be mounted behind the front axle, for better weight distribution and handling.

INTEGRATED AIR-INTAKE SYSTEM
In order to meet the S2000 engine's performance requirements, the intake system has to be capable of moving a large volume of air. In order to achieve this, Honda engineers placed the entire intake system in the space in front of the engine (made possible by the engine's extreme rearward location in the engine compartment). In this location the system can draw cool air directly from in front of the main-forward engine-compartment bulkhead. Since the system does not sit on top of the engine, it also allows for a lower hood height and better forward visibility.

The actual intake system consists of a large, 5.5-liter expansion chamber, a low-resistance, conical axial-flow air filter and a main resonator (intake-noise attenuator). From the air cleaner, intake air flows in a short, direct path to four large-section, tuned intake runners. The runners, along with the cylinder-head intake ports, have been carefully angled in order to provide the shortest, straightest airflow path into each cylinder.

MULTI-POINT PROGRAMMED FUEL INJECTION
The fuel-induction system uses Honda Multi-Point Programmed Fuel Injection (PGM-FJ). PGM-FI is a timed, sequential system with sensors for throttle position, coolant temperature, crankshaft angle, intake-manifold pressure, atmospheric pressure, intake-air temperature, vehicle speed and exhaust-gas oxygen content. Information from these sensors is fed to an Electronic Control Module, which then decides when to activate each injector. PGM-FI can alter fuel delivery to match the engine's needs under varying environmental and engine-load conditions.

LOW-EMISSION VEHICLE (LEV) TECHNOLOGY
Honda engineers designed the S2000 engine to have a high power output and also to be a Low-Emission Vehicle (LEV) engine. In order to accomplish this goal, they installed a new high-flow, metallic honeycomb catalyst and new secondary air-injection system.

An ECM-controlled electric air pump feeds fresh air into the exhaust secondary air-injection system. This allows for very quick heating of the catalytic converter and low exhaust back pressure.

METALLIC HONEYCOMB CATALYST
The S2000 exhaust system uses a new type of thin-walled, low-heat radiating metal honeycomb catalytic converter in place of the more traditional ceramic unit. This new design not only increases the exhaust-gas processing surface area, but also promotes a quick rise in temperature that greatly reduces hydro-carbon emissions during engine warm-up. The catalyst's low back pressure also contributes to better engine performance. The catalyst is also a simpler design, with fewer parts than previous designs.

LOW-RESTRICTION EXHHAUST SYSTEM
The S2000's engine features a low-restriction, high-efficiency exhaust system. The manifold uses large-diameter stainless-steel tubing, and is a 4-into-2-into-1 design that promotes efficient gas flow. A pre-chamber and two main silencers that utilize a "U-turn" pipe arrangement further reduce back pressure.

FLOATING EXHAUST-MANIFOLD COVER
The exhaust-manifold heat shield is held in place by a metal strap that completely surrounds the manifold. Alumina insulators attached to the back of the cover help minimize vibration and noise transmission.

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