43 VPC, VLTC, VPLC and VET systems

84



The Motor Vehicle

Deflection of

camshaft



No timing

change on

drive side



Camshaft

advanced



Crankshaft

rotation



Valve lift



Fig. 3.26 To vary the phase of the inlet cams, the camshaft can be deflected

horizontally, thus causing the half-speed wheel to roll along the belt



Crankshaft rotation, deg

(a)



Torque gain,

Torque gain,

phase retarded

phase advanced



Torque



Torque

Terque gain,

gain, phase

phase

advanced

advanced



Induction

phase

advanced



Induction

phase

retarded



Engine rev/min

(b)



Fig. 3.27 With the variable phase control system (VPC), the inlet opening point can be

retarded to reduce the overlap which, during idling, ensures stability and low

emissions. For good low-speed torque and low fuel consumption, the inlet opening

should be advanced, to increase the overlap



85



Valve lift



Constructional details of the engine



Crankshaft rotation, deg



Fig. 3.28 Stepped cams provide variable lift and timing (VLTC) with rockeractuated valves. At the lower lifts, friction is reduced, charge swirl enhanced

and fuel consumption improved. The curves represent three steps, though

up to ten are practicable



combining these two is to have axially stepped cams, the variation being

effected by shifting the followers from step to step, but such a mechanism is

complex. Tapered cams, such as the Fiat Tittolo system combining variable

lift and event timing, are an alternative but this means virtually point contact,

between cam and follower and, if the duration of opening is kept constant,

the cam is extremely difficult to manufacture. Moreover, the axial loading

introduced is about 10% of the force between the cam and follower, so a

powerful controller is needed.

Honda have a commendably simple system in production, Fig. 3.29. It

has three cams and rockers per pair of valves in a four-valve head. At low

speed, only the outer pair of rockers actuates the valves, leaving the central

one idling freely. As the speed increases, the electronic control signals a

hydraulic valve to open, to allow oil pressure to move a plunger which locks

all three rockers together. In this condition, since the central cam lobes are

bigger than the two outer ones, the latter idle and the former actuates the

valves.

Varying the valve event timing (VET) is the changing of the duration

of lift while keeping the timing and magnitude of maximum lift constant,

Fig. 3.30. In other words, only the opening and closing points are varied.

This improves part load emissions and economy, leaving the wide-open

throttle condition unchanged, and has the advantage that the ramps on the

cam remain effective both as the valve begins to lift and when it re-seats.

VPC and VLC can be combined (VPLC), and it is possible, as in for example

the Mechadyne–Mitchell system, to combine it also with VET (VPET).



86



The Motor Vehicle

Low rev/min



High rev/min



Hydraulic plunger

assembly



Oil in

(a)



Torque



Full load



ECU

change-over

Light load



Rev/min

(b)



Fig. 3.29 (a) Honda variable valve timing system. In the section on the left, the two

outer rocker arms are actuated by their cams, while the central one is idling. Shown

on the right is the condition when electronic control unit (ECU) has signalled a

solenoid to open a valve allowing oil under pressure to push the three-piece hydraulic

plunger to the right, locking all three together. In this condition, the central cam,

because it is higher than the other two, actuates the valves. In the graph (b) the central

curve shows how the ECU varies the change-over point with speed. The upper lines in

the two other pairs of curves represent operation with and the lower ones without

change-over.



3.44



The Mechadyne–Mitchell system



The Mechadyne-Mitchell principle is applicable to almost any of the commonly

used valve actuation mechanisms. Although it entails additional parts, almost

all are identical for each cylinder, so the extra tooling costs are not unreasonably

high. Basically, a hollow camshaft is driven by a peg on the outer end of a

lever projecting from a driveshaft carried coaxially within it. This peg projects

into a slot in the camshaft, Fig 3.31. Axial location of the driveshaft is

similar to that of conventional camshafts.

The arrangement is shown in greater detail in section BB of Fig. 3.32,

from which it can be seen that the peg is actually a ball and slider reciprocating

in a slot in a collar on the camshaft. The driveshaft is moved laterally to vary

the drive from concentric to eccentric. When driven concentrically, as at (a)



87



Valve lift



Constructional details of the engine



Crankshaft rotation, deg



Fig. 3.30 Curves of variable event (VET) timing without phase change. Valve lift

remains constant and the variation is effected continuously. Part-load emissions and

economy are improved, and the wide-open throttle condition is unimpaired.

Characteristics of a VET system with phase change are illustrated in Fig. 3.34



2.4″



2.4″

(a)



1.9″



(b)



Fig. 3.31 Diagram illustrating the principle of the eccentric drive of the Mechadyne–

Mitchell system, (a) in the co-axial and (b) eccentric position, in which rotation

accelerates over about 25° each side of the nose of the cam, thus drawing together the

valve opening and closing points, as in Fig. 3.34



in Fig. 3.31, the speed of rotation of the hollow shaft and cams is constant at

any given engine speed. If it is moved, say, 5 mm off centre, as at (b), its

instantaneous speed of rotation is multiplied in the ratio 2.4 :2.9 = 1.263 : 1.

Therefore, as the shaft rotates, the ratio reduces progressively first to 1:1 at

90°, and then on to the inverse of 2.4 : 1.9, at 180°, and finally back again

to complete the 360°. As the control is moved to increase the eccentricity of

the drive shaft, the duration of valve lift is progressively reduced because its

opening is retarded and its closing advanced.

Appropriate phasing of each cam relative to the eccentric is rendered



88



The Motor Vehicle



Ballpin drive



Drive slot in hollow

camshaft bearing flange



Actuation shaft with

integral eccentric



Dowel



Actuator shaft

in its bearing



Control slide



Section B - B



Driveshaft in its bearing

Section A - A



Fig. 3.32 Sections AA and BB taken from Fig. 3.33 to show in detail the ballpin drive

and actuation shaft

Run of actuator shaft

Run of cam drive shaft



No. 2 cap removed



(a)



(b)



Fig. 3.33 Mechadyne–Mitchell system installed on a six-cylinder engine



possible by dividing the hollow camshaft, along its length, into the same

number of sections as there are cylinders. Each section is carried in its own

bearings and driven by a separate lever and peg projecting radially outwards

from the one-piece drive shaft. Incidental advantages of dividing the hollow

camshaft into short sections are that the ramps on the cams are always fully

effective, and the short lengths of shaft are inherently very stiff both torsionally

and in bending.

Radial movement of the solid driveshaft is effected by the mechanism

illustrated in the plan view of the 24-valve DOHC head of a six-cylinder

engine, and section BB in Figs 3.32 and 3.33. This shaft is conventionally

driven by a wheel mounted on the flange at the left-hand end of the cylinder

head. To one side of, and parallel to, the coaxial drive shaft and six-piece



Constructional details of the engine



89



camshaft is a control shaft. This shaft is actuated by a lever on its end

projecting from the right-hand end of the cylinder head assembly. In a

production version, the controller and actuator would presumably be contained

within the bounds of the cylinder head assembly.

From section BB it can be seen that a mechanism comprising an eccentric

in a scotch yoke slides vertically in the two-piece casting that forms not only

the housing but also the driveshaft bearings and caps. Rotation of the control

shaft moves the whole assembly laterally, and therefore the driveshaft into

and out of concentricity with the camshaft sections. The control shaft can be

rotated through only 90° which is why, when it has been actuated to bring the

driveshaft into its eccentric position, as in section BB, there is no clearance

above the scotch yoke.

By arranging the belt or chain drive as in Fig. 3.26, lateral movement of

the driveshaft can be made to cause also a phase change. The resultant

changes of the valve timing are illustrated in Fig. 3.34, which shows what

the lift characteristics are with the standard timing, what they would be if

only event timing were changed and what it is when both event timing and

phase shift are applied. The whole system can be applied to both the inlet

and exhaust valves but, for optimum cost-effectiveness, only the inlet valve

timing would be varied.

The operating envelope is of course limited by the stresses superimposed

on the valve train by the accelerations due to advancing and retarding the

timing. However, if the eccentricity of the drive is introduced only at speeds

of 2000 rev/min and below, the total stresses in the valve train need be no

higher than those with fixed valve timing at 4000 rev/min. Tests with twin

cylinder motorcycles have shown 32% increases in power and 43% reductions

in specific fuel consumption.



3.45



Control of the Mechadyne–Mitchell system



The friction to be overcome to control the mechanism, by moving the shaft

eccentrically, is not great. Consequently, either electric or hydraulic power

0.4

0.35

0.3

Valve lift, inches



Modified valve

lift with no

phase shift



Modified valve lift

with no phase shift



0.25

0.2

0.15



Modified valve

lift with phase

advance



Modified valve lift

with phase advance



Standard valve lift



Standard

valve lift



0.1

0.05

300 320



340 360 380 400 420 440 460



480 500 520 540 560



Crank angle, deg



580 600 620



Bdc



Fig. 3.34 Characteristics of the Mechadyne–Mitchell system with phase change



90



The Motor Vehicle



could be used. Where hydraulic power is available, for example on power

steered vehicles, this is more attractive, since it does not put any extra load

on the battery. Alternatively, power might be taken from the engine lubrication

system though this would probably entail the introduction of a larger pump

and a hydraulic accumulator, to avoid oil starvation of the engine bearings.

Variations of viscosity with temperature, however, could present problems.

A low-cost alternative would be an on–off solenoid control, possibly based

on only engine speed sensing. The more upmarket models and commercial

vehicles, however, might have a more complex continuously variable

eccentricity controller, with speed, load and VET position sensing for closed

loop mixture control by the ECU.



3.46



Multi-valve heads



Three valves per head, two inlet to facilitate breathing, have been used,

though not widely, for many years. A typical three-valve arrangement, with

a toothed belt driven single overhead camshaft, Fig. 3.35, is that of the

1342 cm3 engine for the Rover 200 range. Although four valves per head

have been virtually universal in aero engines of the reciprocating piston type

since the First World War, and common in racing cars, it was not until the

beginning of the nineteen-eighties that this layout began to be adopted for

engines for upmarket saloon cars. The obstacle, of course, was the complexity

and cost, which outweighed what in the days of low-rated engines were only

slight advantages. In general, two valves are adequate for engines developing

up to about 35 kW/litre but above this level of specific output four are desirable.



Fig. 3.35 The Rover 200 Series 1.3-litre four-cylinder engine has three valves per

cylinder, in a double pent-roof combustion chamber, and a single overhead camshaft

with two rocker shafts



Constructional details of the engine



91



With four valves a greater proportion of the head area is available for

porting than if only two are employed. Moreover, the sparking plug can be

more easily positioned very close to the centre of the chamber, so the flame

path is short. This means that not only is complete combustion easier to

attain but also the ignition timing can be retarded so that the dwell of the

gases at high temperature in the cylinder is reduced, with a consequent

diminution of the NOx content of the exhaust (Chapter 14). Because of the

large diameters of two valves relative to the bore of the cylinder, the gas flow

past the portions of their edges adjacent to the cylinder wall tends to be

masked by it. With four valves, on the other hand, there need be little or no

masking and moreover the interaction of the two incoming streams of gas

can greatly improve mixing, again leading to more complete combustion.

Furthermore, with two exhaust valves instead of one, the ratio of seat length

to area exposed to the hot gases is higher and so also, therefore, is the rate

of cooling by conduction through the seats.

Advantages of the straight inlet port arrangement of the Renault 1.5 litre,

turbocharged V6 engine, Fig. 3.36(a), include not only the fact that it gives

a clear downward path for the incoming gases to follow the receding piston

but also swirl around a horizontal axis can be induced in the cylinder which,

bearing in mind the action of the piston as it subsequently rises, can lead to

exceptionally good mixing. On the other hand, the more commonly used

arrangement, typified by the Saab head for their 2-litre engine, Fig. 3.36(b),

offers a more compact installation and the prospect of introducing swirl

about an axis coincident with that of the cylinder.



( a)



( b)



Fig. 3.36 The Renault V6 1.5-litre turbocharged engine (a) has four valves per

cylinder and its straight, almost vertical, inlet ports induce swirl about a horizontal

axis in the cylinder. In contrast, swirl can be induced about a vertical axis in the Saab

2-litre unit (b), with four valves per cylinder, by appropriate alignment of the inlet

ports



92



The Motor Vehicle



With pushrod actuation, four-valve heads become complex. However, the

increase in complexity is less significant when applied with the valve actuation

gear on a modern twin overhead camshaft engine than with that of a pushrodactuated unit. A compromise is the three-valve layout (two inlet, one exhaust)

as used by Honda and Toyota. Particularly interesting is the use by Toyota of

one large and one small diameter inlet valve, with a hinged flap in the intake

to deflect all the mixture through the smaller one at low speeds, Fig. 3.37.

This, by inducing high velocity flow through the valves, improves mixing of

fuel and air, and thus the driveability of the car.

Kawasaki has an engine with five valves per cylinder, while several others,

including Honda, are experimenting with eight valves per cylinder in engines

with oval bores the major axes of which are transversely oriented. This

entails the use of two sparking plugs per cylinder and two connecting rods

per piston, so the engine becomes very complex. Advantages, in addition to

good breathing characteristics, include the shortness of the cylinder block

and, by producing engines with cylinders having identical minor axes but

different major axes, a potential for manufacturing economically a range of

engines having a common bore spacing but different cylinder capacities.



3.47



Cylinder head – some overall design considerations



The choice between cast iron or aluminium for the cylinder head is not

simple. Aluminium has the advantages of light weight, high thermal conductivity, and ease of production to close tolerances by gravity or low-pressure

diecasting. On the other hand, aluminium is more expensive than iron, tooling



FLAP



Fig. 3.37 At low speeds, a hinged flap deflects all the incoming air through the

smaller of the two inlet valves in the Toyota 1.35-litre three-valve engine



Constructional details of the engine



93



for large quantity production is costly, porosity in the finished casting can

present difficulties, aluminium is more easily damaged in service and rather

more prone to gasket blow-by failure, corrosion may present problems –

especially where there are copper components in the cooling system – and

heat-resistant valve seat inserts are essential.

Cast iron is inherently stiffer, and therefore contains noise better, and is

cheaper. On the other hand, the labour costs in making the moulds and cores

are higher, and more labour may be required for removing the sand cores,

and for fettling.

In a paper presented by D.A. Parker and R.H. Slee, at Symposium 86,

held by AE plc, some particularly interesting comments were made on trends

in engine design, Fig. 3.38. These authors pointed out that many of the

overhead camshaft engines which came into vogue in the nineteen-seventies

had single ohc valve gear with vertical valves and bath tub combustion

chambers in cast iron heads mounted on cast iron crankcases. Screw type

tappet adjusters were used and twin valve-springs obviated a risk of the



1980



(b)

1970



(a)



1990



(c)

Fig. 3.38 Trend in engine design 1970–1990, as illustrated by Parker and Slee, of AE plc