Variable valve concept description

1. Variable Valve Lift

2. Presentation Overview
General overview of the valvetrain
Introduction of variable valve lift and timing
Valve flow
How VVL effects engine volumetric efficiency
Camshaft terminology
Current VVL technology
Fiat/Chrysler Multiair
Honda V-Tec
BMW
GM
Mercedes Benz Camtronics
Where valvetrain technology is headed

3. Why focus on the Valvetrain?
Three major demands to automakers
Fuel economy
Lower emissions
Performance
Continued pressure put on automakers to develop new technology to meet
these demands
Performance, fuel economy, and emissions are interrelated
Once physical engine parameters such as displacement, cam profile,
and compression ratio are set performance, fuel economy, and
emissions and nearly fixed
Compression ratio for example, is fixed once components are chosen

4. Why focus on the Valvetrain?
Optimum engine operating conditions
Most common engine operating condition
Low engine speeds/throttled air flow
Fluid friction and pumping losses

5. Why focus on the Valvetrain?
Position of the crankshaft and the profile of the camshaft determine the
valve events
Different operating conditions have different ideal valve event
schedules
This is a significant compromise
Example
Compression ratio is limited by range with lowest knock limit-ie WOT
However lower engine speeds have less tendency to knock and can
withstand higher compression ratios

6. Why focus on the Valvetrain?
Technology developed to help improve performance, fuel economy, and
emissions
Variable valve actuation technology
Added control of valve timing, lift, and/or duration
Efficiency improvements
Compression ratio increase possible
Load control-Control airflow into engine without restriction
EGR (exhaust gas recirculation)

7. Camshaft Terminology
Noteable terminology
Opening vs closing ramp
Nose
Base circle
Heel
Lift

8. Camshaft Terminology
What is lobe lift?
The amount of travel from the
base circle of the camshaft up to
the nose of the cam lobe
Lobe lift= (base circle and lobe lift) -base circle

9. Camshaft Terminology Cont.
Duration
The amount of time the valve stays off its seat during the lifting cycle of cam lobe
Measured in of crankshaft degrees
Point of lifter rise where the measurement is taken is important

10. Valve seat: Valve head is sitting on this circular hard disk to maintain a good leak
proof seal when closed. It has 30 or 45 degrees sit angle.
Valve Guide: Made from good quality bronze material for guiding and lubricating
valve stem during engine operation.
Valve spring: High grade steel is used to load valve to close during dwell period of
cam lobe.
Seal: A rubber seal is mounted at the end of valve guide to prevent oil leakage into
cylinder during operation of engine.
Collet: These two semi conical parts are used to lock spring
and valve
Rocker Arm :Rocker Arm is an oscillating lever that conveys radial movement from
the cam lobe into linear movement at the poppet valve to open it.
Push Rod: The valve-in-head engine has pushrods that extend upward from the cam followers to rocker arms mounted on the cylinder head
that contact the valve stems and transmit the motion produced by the cam profile to the valves

11. Valvetrain overview
Function:
The function of the valve train is to transform the rotary motion into linear valve motion in order to
control the air/fuel mix or exhaust gases into and out of the combustion chamber.

12. Valve timing
Basic valve events

13. Valve timing
Late Intake valve closing
Hold intake valve open longer than normal
Valve closes in the compression stroke
Piston pushes air from the cylinder back into the intake manifold
i.e. reducing the displacement of the engine
Air in the manifold is more pressurized
Expansion ratio is increased without increasing the compression ratio
Reduction in pumping losses

14. Valve timing
Early intake valve closing:
Close intake valve mid-way through intake stroke
Less air in the cylinder means less fuel is needed

15. Valve timing
Early intake valve opening
Intake Valve opens during exhaust stroke.
Valve overlap is used to control the cylinder temperature.
Some of the inert/combusted exhaust gas will back flow out of the cylinder, via the intake valve, where it
cools momentarily in the intake manifold.
This inert gas then fills the cylinder in the subsequent intake stroke, which aids in controlling the
temperature of the cylinder and nitric oxide emissions.
Improves volumetric efficiency because there is less exhaust gas to be expelled on the exhaust stroke

16. Valve timing
Early/late exhaust valve closing
Exhaust Valve opens during power stroke allowing cylinder to be almost completely emptied of exhaust gas
Exhaust valve closes during exhaust stroke allowing more exhaust gas in the cylinder which increases fuel
efficiency.
This allows for more efficient operation under all conditions.

17. Valve Flow
The most significant airflow restriction in an engine is the flow through the
intake and exhaust valves
Minimum cross sectional area occurs at the valves
Must consider the pressure drop across the valves

18. Valve Flow
In idealized model of a poppet valve
Two minimum area possibilities
Valve curtain area: A1 = πdℓ
Valve seat area: A2= πd2/4
Low lifts- minimum area is valve curtain area
High lifts- minimum area is valve seat area

19. Valve Flow
Flow coefficient
Ratio of effective flow area to a representative flow area
Cl: Flow coefficient, used with valve seat area
Cd:Discharge coefficient, used with valve curtain area
Representative flow area can be defined using the valve curtain area low
lifts) or valve seat area (high lifts)
Arep = ClAeff = Cl(πd2/4)
Arep = CdAeff = Cdπdℓ

20. Valve Flow
Figure shows experimental results of flow coefficient (Cl) vs lift
Arep = ClAeff = Cl(πd2/4)
Effective flow area increases with lift and
flow coefficient increases with lift
Valve seat area (πd2/4) is constant
Maximum flow coefficient seen

21. Valve Flow
Figure shows experimental results of discharge coefficient (Cd) vs lift
Arep = CdAeff = Cdπdℓ
Valve curtain area
Discharge coefficient slightly decreases
with lift

22. Valve Flow
Mach Number defined as the ratio of the local flow velocity to the speed of
sound
m = V/c
Indicates if you are moving at supersonic speed

23. Valve Flow
Figure shows experimental results of volumetric
efficiency vs intake valve Mach index
Can see that volumetric efficiency is
greater at Mach index ≤ 0.6

24. Volumetric Efficiency
Engine intake system – the air filter, carburettor, and throttle plate (in a SI engine), intake manifold, intake port,
intake valve – restricts the amount of air which an engine of given displacement volume (Vd ) can induct. The
parameter used to measure the effectiveness of an engine’s induction process is the volumetric efficiency, ev .
ma = mass of air inducted into the cylinder per cycle
ma = air induction rate into the cylinder
N = engine speed, Ap = piston area, Sp = av. piston speed
If a,i = a,o (atmospheric air density) :- ev measures the pumping performance of the overall inlet system.
If a,i = inlet manifold air density :- ev measures the pumping performance of the cylinder, inlet port and valve alone.

25. Factors effecting volumetric efficiency
The volumetric efficiency depends on:
Fuel type, fuel/air ratio, fraction of fuel vaporized in the intake system, and fuel heat of
vaporization
Mixture temperature as influenced by heat transfer
Ratio of exhaust to inlet manifold pressures
Compression ratio
Engine speed
Intake and exhaust manifold and port design
Intake and exhaust valve geometry, size, lift, and timings
Some of the variables are essentially quasi steady in nature (i.e. their impact is either independent
of speed or can be described adequately in terms of mean engine speed), or dynamic in nature (i.e.
their effects depend on the unsteady flow and pressure wave phenomena that accompany the time-
varying nature of the gas exchange processes.)

26. Effect of IVO, IVC and engine speed in ev

27. Better Volumetric Efficiency
For good volumetric efficiency, Z ≤ 0.6: the average gas speed through the inlet valve should be less
than the sonic velocity, so that the intake flow is not choked.
If Z = 0.6, average effective area of intake valves, Ai is
If Z = 0.6, average effective area of exhaust valves, Ae is
A smaller exhaust valve diameter and lift (Lv ∼Dv /4) can be used because of the speed of the
sound is higher in the exhaust gases than in the inlet gas flow. Current practice dictates:

28. Current Valve Technology
Types of VVA methods
electrohydraulic
electromechanical
Cam-based actuators

29. Honda VTEC
At high RPMs a solenoid injects more oil into the rocker arm which forces
two pins in the larger central rocker arm outwards.
The pins lock on the central rocker arm into the two flanking rocker arms
With every revolution of the larger central lobe on the camshaft the central
rocker arm opens the valves more, allowing more airflow into the engine
for enhanced performance

30. BMW Valvetronic
Cylinder heads with Valvetronic use an extra set of rocker arms, called intermediate arms (lift scaler), positioned between the
valve stem and the camshaft. These intermediate arms are able to pivot on a central point, by means of an extra,
electronically actuated camshaft. This movement alone, without any movement of the intake camshaft, can vary the intake
valves' lift from fully open, or maximum power, to almost closed, or idle.

31. Nissan VVEL
Ran by electric motor that can change the lift of the intake valve. The
engine always runs with WOT

32. Mercedes Benz Camtronic
The system consists of a camshaft that splits in two, plus a centrally mounted
actuator with two vertical tappets.
The actuator makes them drop into channels cut into the camshaft , which
forces the cam sections to move laterally.

33. Fiat/Chrysler Multiair
Launched in 2009(Fiat) and 2011(Chysler)
System aims to boost power, torque, and fuel efficiency of engine
Electrohydraulic actuator controls pressure of fluid that fills passageway
that connects the intake valves and the camshaft
When pressurized, transmits lift
Can constantly regulate valve lift based on engine operation

34. Fiat/Chrysler Multiair
The system uses several strategies to optimize engine efficiency
LIVO
EIVC
Multi-lift
Full- lift

35. Fiat/Chrysler Multiair
LIVO- late intake valve opening
Used at engine start-up and idle
Results in a higher speed air intake
Better mixture formed
Optimized combustion

36. Fiat/Chrysler Multiair
EIVC- early intake valve closing
Used at engine partial load
Stops undesired backflow into the intake manifold
Traps maximum volume of air in cylinder
Increases volumetric efficiency
Reduces pumping losses

37. Fiat/Chrysler Multiair
Multi-lift
Used at low engine loads, idling, city stop and go driving
Open intake valve twice during each intake stroke
Enhances air turbulence and improves combustion

38. Fiat/Chrysler Multiair
Full Lift
Complete valve opening
Used when full engine power is required

39. The Future of Valve Trains
1. electronic valve train engines(camless engines):
as the engine design is getting involved in the
electromechanical
systems and control, research is running these days
toward fully controlled electromechanical valves,
which will leads to a camless engines, and a better
control in the valves positioning.

40. The Future of Valve Trains
advantages:
.better valve timing control
.reduction of mechanical parts
.reduction of friction caused by rocker arms,cam
shaft,cam belt.
.ability to run engine on a higher speed.
disadvantages:
. more complicated engine control system.
. return spring characteristics limitations

41. The Future of Valve Trains
2. pneumatic forced valve closing:
in any four-stroke engine, the valves must be
pushed open for the required length of time, and
then pushed shut before they get themselves
smashed by the rising piston. The opening is
usually accomplished by pressure on the top of
the valve from the camshaft lobe (or a rocker arm
actuated by the camshaft lobe), while the closing
has traditionally been accomplished by a spring
(or sometimes a pair of springs, a small one inside
a larger one), pushing upwards on a ‘retainer’
which is clipped to the top of the valve, so the
research is seeking to get rid of the mechanical
springs and substitute it with a closing pneumatic
system, in order to give the engine the
opportunity to run on a faster rpm without making
any knocking to the valves, as it is precisely
controlled by the electro pneumatic system, and
others using double rocker arms design to make
he lobe open and close the valve..

42. Conclusion

43. References
http://www.mechadyne-int.com/vva-reference/intake-valve-closing-strategies
http://www.mechadyne-int.com/vva-reference/papers/the-impact-of-variable-valve-actuation-
on-engine-performance-and-emissions.pdf
http://www.bmwforums.info/general-guides-and-how-to-s/4638-bmw-valvetronic.html
http://www.me.sc.edu/research
http://www.google.com/patents/US8726861