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Variable Valve Timing

Timing the movement of the valves.

by Julian Edgar

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This article was first published in 2002.

What exactly is infinitely variable valve timing? What advantages does variable valve timing have? Is it the same as an engine having variable valve lift? Let's have a look at the technology, and also why it is advantageous.

The Basics

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The time that the valves open and shut is really optimised for only one engine speed - away from those rpm, fixed camshaft timing is a downer. But what's going on inside an engine to need all of this?

Firstly, when the piston is at Top Dead Centre (TDC) - that's as high up the bore as the piston will go - the inlet valves opens. With the inlet valve open, an air/fuel mixture is drawn into the cylinder as the piston then descends. This is called the Intake Stroke. When the piston reaches the bottom of the bore at Bottom Dead Centre (BDC), the intake valve closes, trapping the air/fuel mixture inside the cylinder. The piston then starts to move back up the bore, compressing the air/fuel mix as it does so. This is called the Compression Stroke. At or near to Top Dead Centre, the spark plug fires. The resulting controlled explosion pushes the piston down with great force, producing the Power Stroke. When the piston reaches the bottom of its travel at Bottom Dead Centre, the exhaust valve opens. The piston then moves back up the bore, pushing the exhaust gas out through the open exhaust valve. This is called the Exhaust Stroke.

Valve Timing

Knew all of that already? OK, but then let's add the next step - the accurate timing of these events. In fact the timing isn't quite as is described above - and it's those 'slight' variations that make all the difference!

It can all get very complicated, so we'll try to keep it as tight as possible.

1. Inlet Valve Closing

The inlet valve usually closes quite a few crankshaft degrees after BDC, when the piston is already on its way up the bore. But why? If the inlet valve is still open and the piston has started heading upwards, you'd expect lots of the air/fuel mixture that had already been inhaled to be pushed out through the open inlet valve, wouldn't you? It is very important to understand why this doesn't happen. When the Intake Stroke is occurring, the air/fuel mix is rushing into the cylinder extremely quickly. In fact, in well-designed engines it's piling through the intake ports at about 800 km/h. The reason that it's rushing in is that the descending piston has created a partial vacuum in the cylinder and the air/fuel mix is flowing in to fill this vacant space.

This torrent of moving air has mass - and so momentum. This means that it keeps on flowing when the piston has stopped travelling downwards, and even when it has started on its way up again! However, as you'd expect, this situation doesn't go on forever - at some point, the rising piston will start pushing the air/fuel mix out through the intake valve. When this process occurs, it is called reversion. The best cylinder filling occurs when the intake valve closes sufficiently late for a touch of reversion is allowed to occur.

2. Exhaust Valve Opening

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So if the intake valve doesn't close until after BDC, what about the exhaust valve which earlier we wrote opens at BDC? If you guessed that it doesn't, you're right. In fact, the exhaust valve opens before BDC is reached. The piston is descending on its power stroke, hot expanding gas pushing it down to develop power and push the car along. So why would you open this valve early and let some of that pressure on the piston escape?

To understand why the exhaust valve opens a little before BDC it helps to think of what the next stroke is. It's the exhaust stroke, where the piston is travelling upwards, pushing the exhaust gas out through the open exhaust valve. To push this exhaust gas out requires power, power that's taken from the crankshaft and so lost from the engine's output. Opening the exhaust valve when there is still some residual pressure in the cylinder allows some of these exhaust gases to escape before the piston starts its upwards stroke. With some of the exhaust gases already gone, the pumping losses that occur as the piston rises (with less exhaust gas to push out of the cylinder) are reduced.

3. Overlap

So the piston is on its way up, pushing out what remains of the exhaust gas after blowdown. When it gets to TDC, the exhaust valve will shut and the intake valve will open, right? Not quite! The exhaust gas is rushing out of the exhaust valve as the piston rises. Just like the intake air stream, this rush of gas has momentum. That means that it keeps going, even after the piston has reached the top of its travel and started to descend. As a result, its closure can be delayed until after TDC.

But we want to make sure that as much fresh air/fuel is breathed in during the Intake Stroke as possible. After all, it is the burning of that mixture which is going to develop the pressure on the piston crown! To accomplish the best inhalation, the intake valve is opened before TDC. That's right - both the exhaust valve and the inlet valve are open at the same time! This is called the overlap period.

So why doesn't the rising piston push the exhaust gases straight out through the open intake valve? After all, it's pushing the exhaust gas out past the exhaust valve... One reason that it does not is that a well-designed set of extractors (or to a lesser extent, any good exhaust manifold) will have a low pressure, scavenging wave arrive during the overlap. This will help draw exhaust gases out through the exhaust valve faster than they would otherwise travel. The rapid flow of exhaust gases out through the exhaust valve will also keep going through its own momentum, creating a low pressure in the cylinder which is filled by the fresh air/fuel mix flowing in through the inlet valve.

So the exhaust valve shuts a little after the inlet valve opens, which in turn stays open a little longer than you'd initially expect it to.....and then you go back to the beginning of the process again.

The Events

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The time at which the intake valve closes is the single most important aspect of cam event timing, but all the events are important. For example, the correct timing of the exhaust valve opening is a trade-off between losing some of the power-producing gas pressure of the power stroke and reducing some of the pumping load, and the amount of overlap will dramatically affect how the engine idles and develops power.

In engines with conventional valve timing, the relationship between the timing of the valve events is fixed. On single cam engines it depends on the shapes of the hunks of metal on the shaft, while in double camshaft engines the relative timing is fixed by the angular relationship of the cams to each other (eg if you jump a tooth on one cam in a DOHC engine the overlap will instantly be fixed at a different amount!). Since the timing of valve events depends a lot on the momentum of the gasflows - and the faster they're travelling the more momentum that they'll have - being able to alter valve timing in relation to speed has great advantages. All those flows in and out can be aided along at all engine loads and speeds, rather than being best for just one operating condition. The results is more torque being able to be developed at all rpm, which in turns means a broader power band.

Variable Valve Timing

There are plenty of ways in which variable valve timing can be achieved, but the actual details of the mechanical operating mechanisms are less important than the overall system.

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Up until recently, most variable camshaft timing has been on only one of the two camshafts. Further, that variation in camshaft timing has often been a single step. That is, when the engine reaches a certain threshold of rpm and/or load, the ECU moves the camshaft timing - so one cam is either in the advanced or retarded position. (The pictured previous model Falcon VCT six-cylinder was only SOHC, so it moved the total camshaft timing in that one step! Not all that useful.)

In many DOHC engines with this type of single variable camshaft timing, it is the exhaust camshaft that is altered in its timing (so perhaps there is a move away from the traditional view away that it's intake cam timing which is most important), although in other engines the intake cam is the one altered.

An example of this sort of variable timing can be found on some Porsches. On one 911 model, the VarioCam system was used to advance the intake camshaft timing by 25 crankshaft degrees when the engine reached 1300 rpm. This resulted in better combustion chamber filling and scavenging, which provided improved torque. When engine speed reached 5920 rpm, the intake camshaft timing was retarded by 25 degrees (ie back to the original idle setting) for optimal high-speed operation. When engine oil temperature was high, the camshaft advance took place at 1500 rpm.

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Single step variable camshaft timing is certainly better than completely fixed cam timing, but it is still relatively clumsy: the engine has only two conditions for which the cam timing is optimised. Obviously much better is to have the cam timing - even of one cam - variable in very small steps. This is termed 'infinitely variable' cam timing. However, wouldn't it be even better still if both camshafts could be infinitely varied in their timing? That way the camshaft timing of both cams could be in almost constant variation, suiting the engine operating conditions at all times.

BMW were the first to use this approach, dubbing it 'double VANOS'. (Their single VANOS system, logically, worked on only one cam.) BMW engines with double VANOS can alter each camshaft's timing by up to 60 degrees! However, in the M5 V8, the inlet cam is altered by 'only' 54 degrees and the exhaust cam by 39 degrees. As you can imagine, this allows quite radical cam timing figures. Overlap, for example, can vary from a massive 80 degrees to negative 12 degrees - the latter where the exhaust cam closes 12 degrees before the inlet opens!

Variable Valve Lift

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Honda's VTEC system is probably the best known for a system where valve lift as well as timing is changed. In the Honda system the camshafts have two extra, "hot" cam profile lobes and two extra rocker arms. The rocker arms for the high lift and duration cam lobes "freewheel" at low rpm - their movement is not transferred to the valves. However, at a certain (usually high) rpm, an electronically controlled and hydraulically actuated pin locks all three rocker arms together, resulting in the hotter cam profile coming into play. The change in camshaft timing and valve lift therefore occurs in one step - the mild engine becomes a rocket.

The Toyota 1.8-litre 2ZZ-GE engine (as used in the current Celica) is another that uses variable valve timing and lift. Toyota's variable valve lift system operates on both the inlet and exhaust valves, with the switch to the high-lift camshaft settings occurring at 6000rpm. The high-lift cam lobes increase intake lift by 54 percent to 11.2 mm and exhaust lift by 38 percent to 10.0mm.

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The high-lift cam profiles have the effect of increasing valve-opening duration, and therefore the range of inlet timing variation. Valve overlap can vary between 4 degrees (full-retard inlet setting and low-speed lift settings) and 94 degrees (full advance inlet and high-speed lift settings). A valve overlap of 94 degrees would normally be associated with full race engines. For comparison, superseded Celica's 5S-FE engine had just 6 degrees of valve overlap and the sports two-litre 3S-GE engine in the first front-drive Celica model had 14 degrees of overlap.

Inlet camshaft timing is varied according to engine revolutions, throttle position, inlet camshaft angle, engine coolant temperature and intake air volume. Inlet cam timing is set to the maximum retard position for engine start-up, operation at low engine temperature, idle and engine shut-down. A locking pin in the controller locks the camshaft timing in the maximum retard position for engine start-up and immediately after start-up (until oil pressure is established) to prevent any knocking noise.

Celica's VVTi system can vary inlet camshaft timing over a range of 43 degrees relative to crankshaft angle. However, the variable lift system has the effect of increasing valve opening duration, so the full range of inlet timing variation is 68 degrees. (Taken from the maximum retard intake valve opening in the low-medium engine speed range at minus 10 degrees BTDC to the maximum advance intake valve opening in high engine speed range at 58 degrees BTDC.)

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The variable valve lift uses a cam changeover mechanism to increase the lift of the intake and exhaust valves when engine revolutions exceed 6000rpm. The hydraulically activated variable-lift mechanism is electronically controlled by the engine ECU and shares some of its hydraulic control hardware with the VVTi system. It has the same system inputs as the VVTi system - crankshaft angle and revolutions, air flow, throttle position, inlet camshaft angle and engine coolant temperature. The variable-lift system will not operate until coolant temperature reaches 60 degrees.

The mechanism includes camshafts with two sets of cam profiles, for low-to-medium engine speed and high engine speed (high lift).The full system includes eight rocker arms (one for each pair of valves), two rocker shafts (located inboard of the camshafts) and a spool-type oil control valve on the aft end of the inlet camshaft.

While these single-step variable lift and timing systems allow for great specific power figures to be quoted, in real life they are pretty crude - such changes in engine torque in one step would never be tolerated in a turbo engine, for example.

Variable Valve Lift AND Timing

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A few other manufacturers vary valve timing and lift in one step in a way similar in outcome to Honda and Toyota, the biggest recent breakthrough in variable valve lift has been BMW's Valvetronic system.

This technology steplessly alters both the timing of each camshaft and also the lift of the inlet valves. The most radical aspect of the new system is that no throttle butterfly is used. Instead, the engine changes its volumetric efficiency by altering the amount of intake valve lift. The Valvetronic system is based on BMW's established double VANOS system, which steplessly varies the timing of both the inlet and exhaust cams. However, the Valvetronic system adds variable valve lift to the inlet cam, achieved by the use of a lever positioned between the camshaft and the inlet valves. An additional eccentric shaft alters the lever's distance from the camshaft, with the eccentric's position determined by a worm drive from an electric motor. The position of the lever converts the cam action into a smaller or larger valve lift, as requested by the engine management system.

So while this system is still one step short of controlling steplessly the lift and timing of all the valves (lift variation is possible only on the inlets), it is the most sophisticated system available - short of electronically or pneumatically controlling the action of the valves individually.

Conclusion

Any 'variable' ability in valve timing or lift can be used to better suit the breathing capability of an engine to the job it is performing at any one time, resulting in improved performance. The more finely that adjustments are made, and the more valves on which these alterations act, the better will be the end result.

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