HONDA SOHC VTEC

Honda SOHC (Single Over Head Cam) VTEC. As popularity and marketing value of the VTEC system grew, Honda applied the system to SOHC engines, which shares a common camshaft for both intake and exhaust valves. The trade-off was that Honda's SOHC engines only benefitted from the VTEC mechanism on the intake valves. This is because VTEC requires a third center rocker arm and cam lobe (for each intake and exhaust side), and in the SOHC engine, the spark plugs are situated between the two exhaust rocker arms, leaving no room for the VTEC rocker arm. Additionally, the center lobe on the camshaft can only be utilized by either the intake or the exhaust, limiting the VTEC feature to one side.

However, beginning with the J37A4 3.7L SOHC V6 engine introduced on all 2009 Acura TL SH-AWD models, SOHC VTEC was incorporated for use with intake and exhaust valves. The intake and exhaust rocker shafts contain primary and secondary intake and exhaust rocker arms, respectively. The primary rocker arm contains the VTEC switching piston, while the secondary rocker arm contains the return spring. The term "primary" does not refer to which rocker arm forces the valve down during low-RPM engine operation. Rather, it refers to the rocker arm which contains the VTEC switching piston and receives oil from the rocker shaft.

The primary exhaust rocker arm contacts a low-profile camshaft lobe during low-RPM engine operation. Once VTEC engagement occurs, the oil pressure flowing from the exhaust rocker shaft into the primary exhaust rocker arm forces the VTEC switching piston into the secondary exhaust rocker arm, thus locking both exhaust rocker arms together. The high-profile camshaft lobe which normally contacts the secondary exhaust rocker arm alone during low-RPM engine operation is able to move both exhaust rocker arms together which are locked as a unit.

The secondary intake rocker arm contacts a low-profile camshaft lobe during low-RPM engine operation. Once VTEC engagement occurs, the oil pressure flowing from the intake rocker shaft into the primary intake rocker arm forces the VTEC switching piston into the secondary exhaust rocker arm, thus locking both intake rocker arms together. The high-profile camshaft lobe which normally contacts the primary intake rocker alone during low-RPM engine operation is able to move both intake rocker arms together which are locked as a unit.

The difficulty of incorporating VTEC for both the intake and exhaust valves in a SOHC engine has been removed on the J37A4 by a novel design of the intake rocker arm. Each exhaust valve on the J37A4 corresponds to one primary and one secondary exhaust rocker arm. Therefore, there are a total of twelve primary exhaust rocker arms and twelve secondary exhaust rocker arms.

However, each secondary intake rocker arm is shaped similar to a "Y" which allows it to contact two intake valves at once. One primary intake rocker arm corresponds to each secondary intake rocker arm. As a result of this design, there are only six primary intake rocker arms and six secondary intake rocker arms.

from wikipedia.org




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VTEC ENGINE Category

Honda's VTEC DOHC (Double overhead camshaft)

VTEC system is a simple method of endowing the engine with multiple camshaft profiles optimized for low and high RPM operations. Instead of one cam lobe actuating each valve, there are two: one optimized for low-RPM stability & fuel efficiency; the other designed to maximize high-RPM power output. Switching between the two cam lobes is controlled by the ECU which takes account of engine oil pressure, engine temperature, vehicle speed, engine speed and throttle position.

A Camshaft

Using these inputs, the ECU is programmed to switch from the low lift to the high lift cam lobes when the conditions mean that engine output will be improved. At the switch point a solenoid is actuated which allows oil pressure from a spool valve to operate a locking pin which binds the high RPM cam follower to the low rpm ones.

Double overhead cams control the opening and closing of a cylinder's valves.
1. Intake
2. Compression
3. Ignition
4. Exhaust

From this point on, the poppet valve opens and closes according to the high-lift profile, which opens the valve further and for a longer time. The switch-over point is variable, between a minimum and maximum point, and is determined by engine load. The switch back from high to low rpm cams is set to occur at a lower engine speed than the up-switch to avoid a situation in which the engine is asked to operate continuously at or around the switch-over point.

- Introduced as a DOHC System in the 1989 Honda Integra and Civic CRX SiR models sold in Japan and Europe, which used a 160 bhp (119 kW; 162 PS) variant of the B16A engine.

- The US market saw the first VTEC system with the introduction of the 1991 Acura NSX, which used a DOHC VTEC V6 with 270 hp (200 kW).

- DOHC VTEC engines soon appeared in other vehicles, such as the 1992 Acura Integra GS-R (B17A 1.7 liter engine).

- And later in the 1993 Honda Prelude VTEC (H22 2.2 liter engine with 195hp) and Honda Del Sol VTEC (B16 1.6 liter engine).

- Honda has also continued to develop other varieties and today offers several varieties of VTEC, such as i-VTEC and i-VTEC Hybrid

DOHC (Double overhead camshaft) valve train layout is characterized by two camshafts located within the cylinder head, one operating the inlet valves and one operating the exhaust valves. Some engines have more than one bank of cylinder heads (V8 and flat-four being two well-known examples) and these have more than two camshafts in total, but they remain DOHC. The term "twin cam" is imprecise, but will normally refer to a DOHC engine. Although most more than 2-valve per cylinder heads employ DOHC, some manufacturers still managed to use a SOHC in 4-valve layouts. Honda for instance with the later half of the D16 family, this is usually done to reduce overall costs. Also not all DOHC engines are multivalve engines-DOHC was common in two valve per cylinder heads for decades before multivalve heads appeared. Today, however, DOHC is synonymous with multi-valve heads since almost all DOHC engines have between three and five valves per cylinder

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The Story of Greet VTEC ENGINE

What is VTEC
The Legenery VTEC (Variable Valve Timing and Lift Electronic Control) is a valvetrain system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine. This system uses two camshaft profiles and electronically selects between the profiles. It was invented by Honda R&D engineer Ikuo Kajitani. It can be said that VTEC, the original Honda variable valve control system, originated from REV (Revolution-modulated valve control) introduced on the CBR400 in 1983 known as HYPER VTEC.

Honda's VTec is a enginering system originally designed for their engines to keep their great fuel economy, while giving extra power at higher rpm's. when the vtech is activated, oil pressure within the head increases to add a larger stroke to lobes on the cams, therefore letting more air in and out of the engine. Here the story of greet VTec Engine.

 

VTEC was the first system of its kind, though other variable valve timing and lift control systems have been produced by other manufacturers (MIVEC from Mitsubishi, VVTL-i from Toyota, VarioCam Plus from Porsche, VVL from Nissan, etc).

 

 

A cylinder head sliced in half shows 2 overhead camshafts (One above each of the two valves)

  

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