Breathe deeply and gulp
How the MGF VVC engine really works…
The historical context:
The quotation which heads this article was written by Laurence Pomeroy. His masterwork The Grand Prix Car was published in two volumes in 1949 and 1954, although most of it had previously been printed in Motor magazine. Aided by superlative technical drawings by Cresswell, it remains in many people's opinion the finest example in the English language of clarity of thought and expression as applied to racing car design and engineering. If you are unable to afford the £500 required to purchase one of the increasingly rare second-hand copies, go down to your public library, borrow them, sit down for a few weeks to let it all sink in, and then you will understand what I mean.
Pomeroy's remark is quite clear. If you wish to design a high performance engine then you should use a system of valve actuation which maximises the efficiency with which an engine breathes the fuel/air mixture in order to get more efficient gas flow and hence more power out. Simple example. Drive a standard push-rod MGA and then a well-sorted MGA Twin Cam and you will understand my point even better. The latter just loves running at 6,500 to 7000 r.p.m. because it has bigger lungs and can breathe better.
The MGF is a Twin-Cam-but what’s that?
The first twin overhead can system drawn up in late 1911 and ealy 1912 by an engineering draughtsman Ernest Henry. Born in Geneva in 1885, he had worked on racing power-boat engines and later with Marc Birkigt of Hispano-Suiza in Paris. Around autumn 1911, he teamed up with a group of ace racing drivers and mechanics (L’ Equipe Boillot) who were commissioned by Mr Robert Peugeot to build a car for the 1912 French Grand Prix to be held at Dieppe on 25th/26th June. Driven by Georges Boillot (easily the Fangio, Moss or Senna of his era), the 7.6 litre 1912 Grand prix Peugeot won, averaging 68.51 m.p.h. over the 956 mile two-day, race, beating three 14 litre airship-engined, chain-driven F.I.A.T.S., by the margin of 12 minutes!
The car and its engine were built outside the Peugeot factory (at the Suresnes aero-engine works of Gnome & Rhone), so in a sense, its directly analogous to a present F1 team commissioning Joe Bloggs Engineering in Northants to build a car for them to race. The others in the team may well have had some input into the twin-cam idea, but Henry was the only chap with the expertise to produce the engineering drawings, so he takes the lions share of the credit. One camshaft actuates the inlet valves and the other drives the exhaust valves. Hence the rates at which both can be opened and closed to let more fuel/air mixture in and out of the cylinders can be better controlled.
Achieving greater control over the breathing of the engine enabled Henry to obtain big car performance from a much smaller engine.
The historical importance of this development was vast, which is why I have described it at length. Prior to Henry's design, motor car engines had been of huge size (15, 20 litres and more) and their cylinder combustion efficiency was pretty appalling. His design of four inclined valves per cylinder controlled by twin overhead camshafts lowered the valve gear stresses, enlarged the valve area, halved the swept volume required for a given power output, and improved combustion efficiency (1) probably by an order of magnitude. A refined design also won the 1913 Indianapolis 500 and the 1913 French Grand Prix.
From then on, whilst minor differences occurred, almost anyone building a high performance engine, whether Sunbeam, Maserati, Mercedes, Alfa-Romeo, Ferrari, Jaguar, Aston Martin and many others, all used Henry's basic principles, wittingly or otherwise.
Little had changed between the Henry-designed Peugeot engines of 1912/13 and the engine developed at the M.G. Car Company from 1956 for the M.G.A. Twin Cam which developed 107 bhp. An important variant was used in the famous EX181 record car now in BMIHT Gaydon which was supercharged to develop 270 bhp. The MGF engine is similar but with a much less pronounced angle between the banks of inlet and exhaust valves.
What makes valves work:
Valves have springs covered by small inverted tin-can shaped things called tappets. As the camshaft revolves, the upper and lower points of each lobe contact and press down on the head of each tappet which in turn pushes the valve down to open. Releasing the pressure closes the valve. The point is that the degree to which you can open and close the valves and the precise time intervals at which this occurs, is fixed absolutely by the design of the camshaft.
One design cannot cope with all extremes of motoring. If you want to toddle off down to the supermarket, stop and start at traffic lights, or need an engine which runs at idle speed (800 to 1000 r.p.m.) quite happily whilst you sit and fume in a traffic jam, you cannot expect to get the high speed power which would give enjoyable sports car motoring. Let alone be useful for any form of motoring sport or competition use, where you might need a camshaft designed to begin to bring in maximum power when the engine is doing 3,500 or 5000 rpm. An Fl engine can give maximum power at 10,000 rpm plus!
Engine design issues:
All car manufactures have to balance many factors when designing their engines.
The principles are:
- The ultimate power required.
- Exhaust emissions.
- Idle speed stability.
- Fuel economy.
- Space/Weight & manufacturing economics.
- Purpose for which the car is being used.
Of these, we all know how stringent UK regulations on emissions have become. Whilst it would be unfair to single them out because there are many other factors within this subject, unburnt hydrocarbons are particularly affected by valve overlap.
It is also important to note that engines have become lighter and smaller due to improvements in ateriald and casting technology. Their lower reciprocating mass has resulted in engines which rev more freely and at higher rev limits.
Variable Valve Control:
The concept which Rover uses was first designed and patented by the UK firm AE Ltd (Associated Engineering Group) in the early 1970s. However, its execution in mechanical terms is pure Rover and was desied and patented in the early 1990s as a means of expanding the range of cars capable of using the K Series engine. It was therefore designed before the MGF was developed as part of Rover's shelf engineering activity.
Several other manufacturers including Alfa-Romeo, BMW, Porsche and Honda have developed systems giving more control than a fixed camshaft. None give the range of control patented by Rover.
The Holy Grail of engine design, is to make a small engine feel and perform like a larger one. Which is exactly the process started by Henry 86 years ago.
The mechanics of how it works:
In its simplest form, what the VVC mechanism does, is to speed up and slow down the camshaft so that the length of time the inlet valves are open can be varied without the profile of camshaft changing. Although the drive to the VVC mechanism is at a constant half engine (or crankshaft) speed, the camshaft velocity varies within each revolution in such a way as to maintain an average of half engine speed.
The heart of the MGF VVC lies just underneath the cam cover.
The first obvious thing one sees is that the inlet camshaft is not a single casting as in most engines, but is in four sections. Inlet valves for cylinders 1 and 2 are controlled by lobes on a half cam-shaft driven by aVVC mechanism (the most important components of which are two drive rings) at the front of the engine. Inlet valves for cylinders 3 and 4 are controlled by lobes on a half cam-shaft driven by a second VVC mechanism (incorporating its own drive rings) at the rear of the engine, the drive for the latter is taken from the exhaust camshaft.
Each half camshaft is in two parts, an inner independent shaft and an external shaft on which the cam lobes are located. Each VVC mechanism housing is machined to swiss watch standards and contains a pair of needle roller bearings.
The other thing one notices is a cast hydraulic control unit and next to it, two 1" square plastic coated solenoids. These are driven from the Engine Management System (EMS) and, through a spool valve, control shaft connected to the toothed control sleeve on the outside of each driving ring assembly.
The really ingenious bit is the design of the VVC mechanism. The outer control sleeve is machined such that it is much thinner on one side than the other, (i.e. it's bore is eccentric to it's outside diameter). The sleeve, can be adjusted through about a quarter-turn which causes the pair of drive rings within the sleeve to move outside the camshaft's centre of rotation. As the drive ring rotates within the sleeve, the camshaft drive pin moves from the inside of the radia slot to the outside, depending on whether the drive ring is at the thin side or the thick side. A pin at the end of the external shaft slots into a hole in the face of the drive ring thus transferring the desired amount of movement to the external shaft and hence to the cam lobes.
Having understood how it works, the next question is, how does the VVC mechanism know by how much to alter things as you drive along?
Information from the EMS MAP:
In order to translate the mechanical system into the measurable performance effect described below, the EMS receives information on a continuous basis from non-contact sensors in several parts of the engine.
These tell it the current status of:
- Inlet cam position.
- Crankshaft rotation speed.
- Oil temperature in the hydraulic control unit.
- Inlet manifold air temperature and pressure.
- Coolant temperature at the top hose.
The difference between what the current status of each is and what the software inside the EMS has been programmed to decide the valve opening period needs to be at any particular fraction of a second, is then transmitted through the hydraulically controlled mechanical linkage of the VVC mechanism to the camshaft lobes.
What is the effect?
The software in the EMS ensures that the VVC mechanism allows the length of time during which each inlet camshaft lobe permits each inlet valve to stay open, to be varied as the camshaft rotates. The amount of variation is huge - 75 degrees between 220 to 295 degrees, or about 40 degrees either side of the basic camshaft profile of 260 degrees if you prefer. The amount of precision control is incredible. The EMS can map and if necessary alter, the degree of valve opening required within each single camshaft revolution.
To quote Laurence Pomeroy again: Increased valve overlap (either side of top-dead-centre) and the extension of the total period of inlet valve opening contribute to increased effectiveness for a given valve area.
A musical analogy:
If you take 1 bar of music, the underlying rhythm and hence the overall speed at which it is played can remain the same as hundreds of bars either side of that one. However a good instrumentalist (whether a string or wind player) can vary each note within one bar, and give clearly audible differences in phrasing.
This in its way, is what VVC does. It listens through the EMS sensors to the way you drive the car. The crankshaft rotates at a constant speed (i.e. constant rhythm), yet the inlet camshaft's rotate at variable speeds depending on the instructions given to it by EMS.
The system increases the power output from 120 PS to 145 PS. This additional 25 PS, is available over much more of the rpm range, giving more torque than the standard car. It also increases the maximum safe limit by 8% from 6750 to 7300 rpm.
The driveability of the car is drastically improved because the increased power is available between 4000 and 7000 rpm, which is what you need when accelerating hard to overtake that lorry. Because the system enables the cam profile (and hence volumetric efficiency) to be accurately matched to the amount of welly you give the accelerator pedal, you use less fuel, probably by as much as 5% to 10% depending on your driving style and the journeys you take.
Finally, how can one measure increased fun. Getting from A to B today is an absolute chore. Anything that contributes to Safety Fast is a big plus. Through the VVC system, Rover have achieved this in quite a remarkable way. What a pity that Pomeroy is no longer around to write about it.
(From Safety Fast Magazine, March 1995)