VTEC. VVT-i. VANOS. VarioCam. These are acronyms that you will have seen in various car ads, all denoting variable valve timing. What is variable valve timing? Is it something truly worth having, or is it just as effective as a sporty sticker on your car: it makes you feel like you’re going faster, but doesn’t really do anything?
Well, let us tell you now that variable valve timing is a genuine technological improvement for the internal-combustion engine, boosting both fuel efficiency and power output.
A couple of basic terms: torque is the pulling power that your engine is capable of. If you hold a 1 kg weight 1 meter away from your body (assuming your arm can extend that long), your arm will feel a force of 1 kg-meter, or about 10 Newton-meters (Nm). Power is that force applied over a certain time. Spin an engine with 10 Nm at 1000 rpm, and you’ll generate about 14 bhp (10.4 kilowatts) of power. The faster you spin an engine, the more power it generates. That’s how the same 3.0-liter displacement can generate 155 bhp at 4000 rpm in a Hyundai road car engine, and almost 900 bhp at 19000 rpm in a Ferrari Formula One powerplant. Now some of us would love to be driving around with engines peaking at 19000 rpm, but it’s not practical: we’d all have to be shifting like crazy, cars would be screaming all the time, and we’d need to refuel every 100 laps, er, km.
For the sake of fuel efficiency, modern engines tend to have smaller displacements. They can be made to generate the same power as larger, older engines by revving them higher. However, the wide rev capability creates the problem of which part of the rev range to optimize the valve timing for. A mild or nearly round cam profile is ideal for the lower revs; it creates a low, wide torque band with good emissions and smooth idle. Such an engine has good initial acceleration but runs out of breath at higher revs. If you’ve driven a Lite-Ace, you know how this feels.
Racing engines have wilder or more pointy cam profiles, enabling the valves to open sooner and stay open longer. They produce a prodigious amount of power at high revs. However, they’re somewhat limp at lower revs, and snarl and pop at idle. Until recently, road car engine designers have had to compromise and set their cam profile somewhere between the two extremes. So we’d get engines with average torque, average fuel efficiency, and average performance.
Enter Honda. In the 1980s Honda came out with the first application of variable valve timing in a road car. Instead of one cam profile, the Honda Variable Valve Timing and Lift-Electronic Control (VTEC) engine contained two settings, one for low rpm and one for high rpm. At low rpm, the cam profile was mild, and at a certain changeover point, the more aggressive cam profile would take over. So at a certain rpm, your previously docile Civic would transform into a wild animal.
Honda engines with VTEC have improved both fuel economy (about 8% better than non-VTEC counterpart) and high-end power. VTEC-E is actually the Economy VTEC used in lower-end Civics, used primarily to improve emissions and fuel economy. DOHC VTECs are tuned for high-end power in the Civic SiR, the Civic, Accord and Integra Type-Rs and the S2000: not much happens below 6000 rpm, but rev it past that point and you awaken the monster! You can rev to 9000 rpm, too!
Here’s an illustration of Honda’s new 3-stage VTEC, already in Japanese Civics:
In stage one, for low revs, the engine uses two different cams for the two intake valves. The cam on the left, the nearly round one, just cracks open the left intake valve, while the right cam, with a medium profile, operates the right valve with medium lift. With the left valve just barely opening, the fuel-air mixture is swirled in the cylinder before combustion, resulting in good torque and low emissions.
Stage 2 is for medium revs. Oil, illustrated as orange, is pumped into a cylinder, locking the two outer cams together. Now both valves are made to follow the right cam, for medium timing and medium lift.
At high revs, Stage 3, oil enters both chambers of the VTEC system, thus locking all three pins together. Now both cams are forced to follow the middle cam, which has quick timing and high lift. This generates the top-end power that Honda VTECs are famous for.
The main advantage of VTEC and similar systems like Mitsubishi’s MIVEC and Nissan’s Neo VVL is that they can change both the cam timing and the valve lift resulting in maximum power at the top end. However, low-end torque does not improve much.
This is where VVTi comes in. Unlike VTEC, VVTi does not use dual cam profiles and doesn’t have a Dr.Jekyll-Mr.Hyde changeover point. Rather, it varies the intake valves’ timing continuously throughout the rev range. At higher revs, it opens the valves earlier to promote better breathing. Thus the valve timing changes. However, the valve lift or how long the valves stay open, doesn’t change.
In a VVTi engine, the end of intake camshaft incorporates a gear thread. This thread is coupled with a cap that can move towards and away from the camshaft. Because the gear thread is not parallel to the axis of the camshaft, valve timing will shift forward if the cap is pushed towards the camshaft. Similarly, pulling the cap away from the camshaft results in shifting the valve timing later. Hydraulic pressure controlled by the ECU pushes or pulls the mechanism depending on engine rpm and other conditions.
The “i” denotes that the ECU adjusts valve timing not just based on engine rpm but on several factors including vehicle acceleration or deceleration, uphill or downhill direction, etc.
BMW’s double-VANOS system uses a similar mechanism on both the intake and exhaust valves. Ferrari, Alfa Romeo, Porsche, Ford, Jaguar, Lamborghini and Renault all use similar systems on their current engines.
VVT-i and VANOS are theoretically simpler than VTEC because there is only one mechanism per camshaft and not each individual cylinder. However, VANOS systems have been known to need repair after 100,000 km or so.There have been no recorded warranty claims on VTEC engines, despite their high-rev capability. VVT-i is relatively new, so its durability factor is still unknown. Given Toyota’s reputation, however, we doubt that anything will go wrong with these engines.
Again, Toyota and Honda have remained true to their identities: Toyota has used its variable-valve system to improve low-end torque and everyday drivability. Honda chose a system that would result in a sportier engine: maximum top-end power. Toyota won’t sit still, though, and they have already introduce the VVTL-i, which varies valve timing and lift, similar to the VTEC system. The advantage goes to VVTL-i, as it can vary valve timing continuously. The new Celica GT-S has a 1.8-liter VVTL-i that can pump out 180 bhp, or 100 bhp/liter, a match for Honda’s DOHC VTECs.
The future may lie in electro-magnetic rather than mechanical valve actuation, so these variable valve systems are only a temporary solution. For now, though, those with a VTEC or VVT-i sticker on their cars can be pleased that those systems are actually helping them go faster while saving more fuel.
With information and photos from Honda, Toyota, Mitsubishi, Porsche, BMW, Alfa Romeo and AutoZine.
By Jason Ang | Information and Photos from Respective Manufacturers
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