Honda Prelude -- Drivetrain

OVERVIEW
The Prelude and Prelude Type SH models retain the 2.2 liter, 16-valve twin-cam engine.

The Prelude VTEC engine is a well-proven power plant, offering compact, efficient power, reliability and durability. In addition, the Prelude VTEC engine serves as an excellent example of Honda engine-building expertise and use of advanced technologies. For example, the engine features a high-performance version of VTEC, the Honda-designed variable valve timing system. VTFC changes intake and exhaust valve timing and valve lift in order to maximize engine torque throughout the engine's rpm range.

Since one of the major engineering goals for the Prelude was an increase in the car's level of refinement, Honda engineers have made numerous improvements to the engine in order to reduce noise, vibration and emissions.

A new Sequential SportShift 4-speed automatic transmission (not available for the Type SH) is lighter and more compact than previous versions and operates more smoothly. In addition, the transmission's electronic control unit uses Honda's Grade Logic Control System.

Grade Logic can determine when a vehicle is driving uphill or downhill, or braking hard into a corner. It then automatically downshifts from 4th gear to 3rd gear in order to eliminate any gear hunting (uphill) and to utilize engine braking (downhill and when cornering).

The automatic's Sequential SportShift feature combines the convenience of an automatic transmission with the fun of manually shifting gears. When the driver wants the transmission to shift automatically, they can leave it in the normal "D" mode. However, when they want to extract the maximum performance from the engine, the transmission can be shifted into sequential mode and the driver can control upshifts and downshifts.

PRELUDE VTEC ENGINE
The Prelude VTEC engine block and head are high-pressure die castings made with aluminum alloy. The block is an open-deck design and has an undersquare bore-stroke ratio with a 3.39-inch (87.0 mm) bore and a 3.53-inch (90.7 mm) stroke. The engine's long-stroke design allows closer cylinder-bore spacing, which helps keep overall engine length down and also yields stronger low-rpm torque. The walls of the block extend below the centerline of the crankshaft, which helps stiffen the bottom end. Additional bottom-end rigidity comes from a massive cast-aluminum bearing-cap carrier. The engine is angled back 10 degrees in its mountings for better weight distribution.

FIBER-REINFORCED METAL CYLINDER LINERS
Instead of cast iron, the Prelude engine block's cylinder liners are made of a metal-matrix composite material Honda calls fiber-reinforced metal (FRM). FRM is a mixture of carbon fiber and aluminum oxide that, when used in cylinder liners, offers several advantages over conventional cast-iron liners. For example, FRM liners transfer heat to the cylinder water jackets more rapidly. This allows engine designers to build a smaller, more compact engine and cooling system. The designer may elect to keep the same size engine, but increase its power output. Honda engine designers elected to keep engine size and power output fixed and utilize FRM's superior heat-transfer capabilities to increase engine durability.

Since FRM is a ceramic-based material (aluminum oxide is a ceramic used for spark plug insulators), it also exhibits higher wear resistance than cast iron. This results in potentially longer engine life.

Finally, FRM liners weigh less than cast-iron liners, thereby helping to minimize engine weight.

Additional examples of the durability Honda engineers have built into the Prelude engine are its gravity-cast, aluminum-alloy pistons and drop-forged steel connecting rods. The pistons receive additional cooling via a set of jets that spray pressurized oil at the underside of the piston crowns--a technique first used by Honda on its Formula-i engines.

The compression ratio is 10 to 1, and premium, unleaded fuel (96 octane RON) is specified. A knock sensor imbedded in the cylinder head detects any incipient combustion knock (detonation) and automatically retards ignition timing for safe operation.

SECOND-ORDER BALANCE SYSTEM
The Prelude engine block also incorporates a Honda-designed second-order balance system. The system eliminates much of the Prelude engine's mid-range and high-rpm vibration.

Four-cylinder engines are attractive to vehicle designers because of their short, compact configuration. This allows them to be fitted transversely into the engine compartment, which frees up more space for passengers and cargo. In addition, short crankshafts and compact cylinder blocks save performance-robbing weight. However, when 4-cylinder engines grow larger than about 2 liters in displacement, the second-order vibration (twice per engine revolution) set up by their pistons and connecting rods, can become objectionable. The second-order balance system built into the Prelude engine effectively counteracts the inertial moments created by its large pistons and connecting rods.

The system consists of two parallel shafts on either side of the crankshaft, 3.19 in. above its centerline. Driven by a toothed belt, the balance shafts rotate in opposite directions at twice engine speed. Eccentric weights built into the shafts generate inertial forces that counteract the second-order forces. The Honda system differs from other balancing systems in that it is designed to minimize vibration in the mid-to-high-rpm range, since this is the area in which the engine operates much of the time.

CAST-ALUMINUM OIL PAN
This latest Prelude VTFC engine incorporates a die-cast aluminum oil pan in place of a conventional steel stamping. In addition to its greater thickness, the new pan is extensively reinforced with internal ribs. As a result, the new pan is extremely effective in blocking crankshaft and bottom-end engine noise and helps to increase the rigidity of the entire engine.

Another source of engine noise and vibration is minimized by a cast-aluminum stiffener. The stiffener connects the oil pan and engine to the transmission, forming a single, rigid unit which is much more resistant to resonance and vibration.

PISTON DESIGN
A piston design featuring a full-floating crankpin also contributes to quieter engine operation. The full-floating design allows for a closer fit between the pin and the piston, thereby reducing any clatter or noise as the engine warms up.

CYLINDER HEAD
The Prelude cylinder head is low-pressure die-cast from aluminum alloy. The individual combustion chamber's pent-roof configuration and centrally located spark plug promote rapid, complete burning of air and fuel.

Each cylinder has 4 valves (two intake and two exhaust). Since the individual valves in a 4-valve combustion chamber are smaller and lighter than the valves in a 2-valve combustion chamber, there is less reciprocating mass. This allows the engine to be revved safely to higher rpm levels, helping to extend the engine's power range. Four-valve combustion chambers also have greater valve area, so they offer less restriction to intake and exhaust flow, better exhaust-gas scavenging and greater volumetric efficiency.

Valve actuation is via dual-overhead camshafts and direct-acting rocker arms located under the camshafts. Direct-acting rocker arms permit the use of screw-type adjusters for easier valve adjustment. The rocker arms also house the variable valve-timing mechanisms. The camshafts are driven by a Kevlarreinforced toothed belt.

The distributor for the high-voltage ignition system is driven off the end of one of the camshafts.

MULTI-POINT PROGRAMMED FUEL INJECTION
The fuel-induction system uses Honda Multi-Point Programmed Fuel Injection (PGM-FI). 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 Unit, 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.

DUAL-STAGE INTAKE MANIFOLD
A dual-stage intake manifold improves low- and mid-range torque. At low- and mid-range rpm, air is drawn through a primary intake tract, which helps keep intake velocity high and creates good turbulence and cylinder filling. As engine rpm increases, a second tract opens at 4800 rpm to satisfy the engine's demand for additional air.

QUIETER, MORE EFFICIENT 4-INTO-2-INTO-1 EXHAUST SYSTEM
The Prelude engine uses a 4-into-2-into-1 exhaust manifold. The manifolds four individual runners improve engine breathing by more efficiently scavenging exhaust gases. The manifold has been redesigned for better flow characteristics and more power. In order to better minimize resonance and vibration in the manifold, the two-pipe header section is shorter and more strongly triangulated.

The Prelude engine's muffler mounting has also been redesigned to better minimize vibration and noise. In addition to the existing attachment at the pipe leading to the muffler, one of the rear attachment points has been moved to the front of the muffler, thereby creating a more strongly triangulated mounting.

A new type of rubber isolator is used to mount the muffler to the body. It is more resistant to resonance and is made of a more durable elastomer.

VARIABLE VALVE TIMING
The Prelude VTEC engine uses a performance version of Honda's innovative variable valve timing system (VTEC stands for Variable Valve Timing and Lift Electronic Control). VTEC maximizes the Prelude 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 burned 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. Since air and fuel have mass, and therefore inertia, inevitably there is a time delay created as the mixture is accelerated and moved by the suction of the piston on its intake stroke. Inertia also creates a second time delay because it keeps the intake charge moving toward the cylinder after the intake valve has closed and the piston has begun its compression stroke. This time delay increases as engine speed increases. At the upper end of an engine's rpm range, the intake valve ends up closing before a significant portion of the air/fuel charge reaches it.

High-performance and racing engines essentially operate at the upper end of their rpm range, so their designers compensate for the intake charge delay by using cam-lobe profiles that open the valves to a greater degree (more lift), and hold them open for a longer duration; however, this creates an entirely new set of problems: At low- and mid-range engine speeds, long-duration, early-opening, high-lift cam timing will keep the valves open too long. As a result, part of the intake charge leaks back out of the cylinder before the intake valve can close. Additionally, residual exhaust gases can leak back into the cylinder and dilute the intake charge. As a result, engine torque will drastically decrease. This is the major reason high-performance racing engines are traditionally so "peaky" and suffer from driveability problems. 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 Prelude 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 5000-5600 rpm, the two intake and exhaust valve rocker arms at each cylinder are actuated by low-rpm cam lobes. Their short duration and low lift ensures good cylinder-filling at low engine speeds. On the intake side, the valve timing is slightly staggered so that one valve begins opening before the other. This creates a swirl effect and greater turbulence as the intake charge enters the combustion chamber, resulting in better combustion efficiency.

HIGH-SPEED OPERATION
At 5000-5600 rpm (depending on throttle position), an electronic control unit commands a spool valve to open and send oil pressure to pins in the rocker arms. Under pressure, the pins lock the two intake-valve rockers and the two exhaust-valve rockers to a third rocker arm. Until this moment, this third rocker arm (there is one on the intake side and one on the exhaust side) has been independently following the contour of a separate high-lift, long-duration cam lobe. Now the valves are actuated by the third rocker-arm follower and more closely match the induction and exhaust timing required for optimum torque at high engine speeds.

ON-BOARD DIAGNOSTIC SYSTEM (OBD-II)
The OBD-ll system used on the Prelude engine expands the control and diagnostic capabilities of Honda Programmed Multi-Point Fuel Injection to include emissions-system components and operation.

If the fuel-system Electronic Control Module (ECM) detects a fault in the emissions system, it can take several different steps to correct the problem.

Depending on the importance of the component or system, it may decide to continue to monitor and test the component to verify that there is indeed a fault and not just an intermittent problem. If it determines that a fault is serious enough, it may immediately light the "check engine" light to alert the driver that service is required. At the same time, it stores a failure code called a Diagnostic Trouble Code (DTC) in its memory for retrieval by a service technician (several DTCs can be stored). This helps make service quicker and easier. If a component failure is serious enough, OBD-ll can even alter engine performance to compensate.

The components of the system include an Electronic Control Module, a charcoal-canister purge-system monitor, an engine-misfire detector and a set of oxygen sensors that monitor the efficiency of the catalytic converter.

The engine-misfire detector uses a magnetic sensor that reads cylinder-firing information from the toothed, cam-drive pulley on the end of the crankshaft. The sensor is capable of reading the minute angular accelerations of the crankshaft that accompany individual cylinder firing. This data is sent to the ECM. If a cylinder is not operating efficiently, the ECM will take appropriate action.

California models are also equipped to detect a fuel-vapor leak in the purge line connecting the fuel tank, charcoal canister and throttle body.

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