Three Stage VTEC
From: Shawn Church
First of all despite being 16 valves there is really only one cam lobe for operation in the first two stages. This is supplemented by the high lift center lobe (like in the normal DOHC VTEC). The second intake valve rocker however simply rides along a zero lift ring in place of the usual lobe on the cam shaft.
To activate the second stage at 2500 rpm(both valves opening) oil pressure is diverted into a channel that drives a pin which connects intake valve rocker #1 with #2. Now both intake valves are being operated in unison. This is perhaps not quite as ideal as a little valve stagger (as seen in the DOHC VTEC apps) but sufficient enough for good power.
To enter the third stage at 6000 rpm oil is now allowed into a second channel connecting all the rocker arms. This oil drives a pin which connects the linked #1 and #2 rockers to the center lobe actuated #3 rocker which is of course the high lift and duration mode.
Frankly the implementation from an engineering standpoint is absolutely brilliant. Youll have to see the diagrams to see what I mean.
Power for the 1.5 peaks at about 130 hp@6900 (had to estimate from a chart) and about 105-107 lb-ft @ 5200. The torque curve is very flat down to about 2000 rpm and it still makes about 80 lb-ft at 1000 rpm. The motor (not necessarily the car) is also apparently capable of VTEC-E type economy. Id have to say that its pretty impressive to take a motor that is 6.5% smaller than the 1.6 and have it make as much torque and more hp with potentially better economy. Putting this technology in the Accord 2.2 with some torque tuning would make for a great everyday car (I figure 160+ hp and 150+ lb-ft).
--------------------------------------------------------------------------------
Two Stage VTEC
From: Daryl Krzewinski (daryl.krzewinski@ab.com)
VTEC is Hondas trademarked acronym for Variable valve Timing and Electronic lift Control. Put simply its a method of directly altering the cam profile that valves see so that the optimum grind can be utilized at either high or low rpm. Honda currently has three different VTEC systems for sale in the U.S. but the primary differences are: level of complexity and design purpose. Since the high performance version is the most oft discussed Ill describe it:
The high performance VTEC system which made its debut in the Acura NSX is also available on the Integra GS-R Prelude VTEC and del Sol VTEC. Using radical cam grinds to improve engine horsepower is certainly nothing new but the problem lies in driveability. The very aspects of a cam grind that work so well for horsepower (high lift long overlap etc.) do so by creating an ideal situation for a high air flow at high engine speeds. Unfortunately what works well for that situation has the inverse affect on low speed torque and driveability. Perhaps at one time or another weve all heard the V-8 hot rod that has a cam grind so radical that it cant even maintain an idle and the driver must constantly goose the accelerator to keep the engine running.
What to do? How about two different cam grinds each optimized for a different half of the rpm range. Honda achieves this with a rather simple method.
Picture if you will one cylinder of a DOHC 4-valve per cylinder engine. There are 4 cam lobes each directly operating a valve (two intake two exhaust). The VTEC system has two more cam lobes in between each pair of the other respective sets. These two can then be our high-rpm lobes while the other four are the low-rpm lobes.
The low-rpm lobes in this case then actuate the valves through a set of rocker arms so that the mechanical connection can be broken if desired. The third high-rpm lobe also has its own follower but it is in a freewheeling state flopping around and not contributing anything. As our engine accelerates through its rev range it passes through the power peak of the low-rpm lobes. Then at the engine speed and throttle position programmed into the computers memory map a signal is sent which electronically opens a spool valve which then directs oil pressure to a mechanical sliding pin. This pin locks the rocker arms actuating the valves to the follower on the high-rpm cam lobe. As this grind is steeper and higher then the other four cams it will supersede them. In a few milliseconds you have completely altered the valve timing and the engines power band begins anew.
The obvious benefits to this are the high-rpm power associated with a radical cam grind but with little or no negative affects on low speed idle driveability or torque. Just changing a fixed timing engine to a cam grind equal to the high-rpm one used in the VTEC would produce an engine which is utterly gutless below 5000rpm.
Clearly this system is intended to improve performance first with little effect elsewhere. But such a system can be used differently as that in the Civic VX. In this instance the low-rpm lobes give a staggered timing where one valve opens fully but the second cracks just a bit. This is to induce a high swirl rate into the chamber to promote better combustion which when combined with a computer-actuated lean burn helps to achieve high mileage. The high-rpm lobes in this engine are a more conventional grind associated with a sixteen valve 4-cylinder to provide extra power in cases of passing or merging. The VTEC system used in most Civics (EX Si and the del Sol Si but NOT del Sol VTEC) is a little closer to that in the NSX etc. The difference being that this system is vastly simplified and operates on the intake valves ONLY. The exhaust valves are actuated conventionally which reduces the effect somewhat from the full VTEC system. This is partly to reduce costs and partly because this engine is a SOHC and the complex system of rocker arms to actuate sixteen valves is prohibitive to the full VTEC system. The VTEC on the Accord is close in design to that on the Civics as well optimized more for a smooth power delivery then high horsepower.
The negative effects? Very few really. Obviously its very expensive with many complex parts involved. The biggest drawback is the limitation to only two modes of valve timing. Most engineers are still seeking ways to obtain unlimited variance of the valve timing so that it can be optimized to any engine speed not just high or low rpm. BMWs system approaches this method with a completely different method of varying the valve timing. It is almost infinitely adjustable *within its range* but alas it has a much smaller envelope between the two extremes of its variability than is possible with the Honda system.
--------------------------------------------------------------------------------
From: rtsang@chat.carleton.ca (Ricky Tsang)
The Honda Civic with VTEC was first introduced in 1992.(USA & Canada). There are two different engines. The VTEC(D16Z6) and VTEC-E(D15Z1). The VTEC is equipped with multiple cam lobes per cylinder providing one valve timing and valve lift profile at low speed and a different profile at high speed. Switch-over from one profile to the other is controlled electronically and is selected by monitoring current engine speed and load. In general it would be ideal if the high rpm performance of a racing engine and the low rpm performance of a standard passenger car engine could be combined in a single engine. This would result in maximum performance engine with a wide power band. Two of the major differences between racing engines and standard engines are the timing of the intake/exhaust valves and the degree of valve lift. Racing engines have longer intake/exhaust timing and a higher valve lift than standard engines. The Honda Variable Valve Timing and Valve Lift Electronic Control System takes this into account. When valve actuation is set for low rpm timing and lift low rpm torque is better than in a standard engine. When valve actuation is then switched for high rpm timing and lift output improves to the level given by a racing engine can offer. Until now few variable valve timing systems have been commercialized. In those that have only the time that both valves are open (intake/exhaust overlap) could be changed. Hondas system is the first in the world in which the intake valve timing and the degree of valve lift can be changed as needed making it the most advanced valve train mechanism available. The Civic VTEC is rated 125bhp @ 6600rpm.
The VTEC-E has a normal 4 valve per cylinder valve arrangement. At low rpm the primary intake valve operates at normal lift while the secondary intake valve opens only slightly to prevent fuel accumulation in the intake port. At high rpm the secondary intake valve rocker arm is connected to the promary intake valve rocker arm to allow normal valve lift A synchronizing piston connects/disconnects the two intake valve rocker arms. Hydraulic pressure against a trming piston moves the synchronizing piston one direction while a stopper piston and return spring moves the synchronizing piston back when hydraulic pressure is reduced. A variable valve timing and valve lift mechanism is used so the engine achieves both low fuel consumption and high output. With this system a very lean fuel/air is efficiently burned to achieve high torque characteristics and low fuel consumption in the low rpm range while in the high rpm range high output equivalent to that of conventional 4-valve engine is achieved. The VTEC-E engine has two intake valves per cylinder. In the high rpm area both intake valves are activated while only one valve operates in the low rpm area. Switching between one valve operation and two valve operation is controlled be hydraulic pressure. The VTEC-E is rated 92bhp@5500rpm.
From: wolsten@infi.net (Daniel E. Wolstenholme)
Theres more to the differences between VTEC and non-VTEC engines than this. In my 94 Integra I read through the service manual to find all the differences between the two engines. The blocks are the same but the cylinder heads (of course) are completely different. Inside the block there are oil jets in the VTEC version different bearing caps and a bearing cap bridge and different connecting rods (this is important) due to higher stresses and the smaller stroke on the VTEC engine. Outside the engine there is an oil cooler and a different intake manifold with a secondary intake path that opens above 5750rpm. In the transmission theres a different final-drive ratio and gears 3-5 are smaller. Looking at the cooling system theres a different thermostat housing and a lot of extra piping in the cooling system (I dont know what its all for--for instance theres something called a heater bypass pipe thats not present in the non-VTEC engine. I also recall some kind of engine stiffener being present on the VTEC model. I suspect that the PGM-FI is different since the engines operate quite differently (one has a much higher redline). The VTEC camshafts as you say are mechanical but they are actuated electronically.
From: Shawn Church
12/21/96
VTEC is actually a generic name for several different systems within the Honda engine lineup. You have the SOHC VTEC which operates only on the intake valves the DOHC VTEC which operates on both and in other countries there is a three stage VTEC system which I believe only comes on DOHC cars (130 hp from 1.5 liters and excellent mileage is the example Im familiar with).
The principle of all the VTEC systems is the ability to change cam profiles at a given point within the rpm range. This allows a cam which will easily idle and pass emissions for everyday driving but provides an extra kick on the top end (it can also be used to improve economy or low end torque).
This change in cam profiles is accomplished through the use of a solenoid and oil pressure. At a given rpm oil pressure is allowed to lock together the normal cam followers and a third follower. This third follower is actuated (normally) by a higher lift and duration cam lobe and is supported be a lost motion assembly. When the followers lock together the two valves being controlled are following the exact same profile (most DOHC motors have a slightly different profile for each valve to improve swirl and combustion).
The implementation is much more complex in the SOHC system because of the need for a very complex actuator system to get around the limitations of a single cam to actuate 4 valves per cylinder.
The actuation of the system does indeed follow a hysterisis loop and the actuation point is variable based upon load and throttle position. Without the hysterisis loop you would have a certain point where the system would rapidly cycle in and out causing problems.
The three stage system actually actuates only one valve at low rpm both valves at mid rpms and then switches to a high lift lobe on the top end. Sounds complex but the implementation is actually elegantly simple.
To sum up there are no additional valves in VTEC systems just a different cam lobe with which to actuate your valves. The only necessary additional parts are the solenoid different cams lost motion assemblies and the ECU to run the system. In general you wont hear the system activate on a stock car. On DOHC VTEC systems what most people hear activating is the secondaries on the variable path intake manifold. To take advantage of the system keep the revs up hopefully high enough so that when you shift you wont drop out of the VTEC range. If you had a DOHC car youd also want to try and stay in the secondary range of the intake manifold.
:::
Creative Draft Image Manipulation Forum:::
now thats technology
