Desmodromic valvegear

Browse pages
Current page

1

Current page

2

Current page

3

Current page

4

Current page

5

Current page

6

Current page

7

Current page

8

Current page

9

Current page

10

Current page

11

Current page

12

Current page

13

Current page

14

Current page

15

Current page

16

Current page

17

Current page

18

Current page

19

Current page

20

Current page

21

Current page

22

Current page

23

Current page

24

Current page

25

Current page

26

Current page

27

Current page

28

Current page

29

Current page

30

Current page

31

Current page

32

Current page

33

Current page

34

Current page

35

Current page

36

Current page

37

Current page

38

Current page

39

Current page

40

Current page

41

Current page

42

Current page

43

Current page

44

Current page

45

Current page

46

Current page

47

Current page

48

Current page

49

Current page

50

Current page

51

Current page

52

Current page

53

Current page

54

Current page

55

Current page

56

Current page

57

Current page

58

Current page

59

Current page

60

Current page

61

Current page

62

Current page

63

Current page

64

Current page

65

Current page

66

Current page

67

Current page

68

Current page

69

Current page

70

Current page

71

Current page

72

Current page

73

Current page

74

Current page

75

Current page

76

Current page

77

Current page

78

Current page

79

Current page

80

Current page

81

Current page

82

Current page

83

Current page

84

Current page

85

Current page

86

Current page

87

Current page

88

Current page

89

Current page

90

Current page

91

Current page

92

Current page

93

Current page

94

Current page

95

Current page

96

Current page

97

Current page

98

Current page

99

Current page

100

Current page

101

Current page

102

Current page

103

Current page

104

Current page

105

Current page

106

Current page

107

Current page

108

Current page

109

Current page

110

Current page

111

Current page

112

Current page

113

Current page

114

Current page

115

Current page

116

Current page

117

Current page

118

Current page

119

Current page

120

Current page

121

Current page

122

Current page

123

Current page

124

Current page

125

Current page

126

Current page

127

Current page

128

Current page

129

Current page

130

Current page

131

Current page

132

Current page

133

Current page

134

Current page

135

Current page

136

Current page

137

Current page

138

Current page

139

Current page

140

Current page

141

Current page

142

Current page

143

Current page

144

Current page

145

Current page

146

Current page

147

Current page

148

Current page

149

Current page

150

Current page

151

Current page

152

Current page

153

Current page

154

Current page

155

Current page

156

Current page

157

Current page

158

Current page

159

Current page

160

Current page

161

Current page

162

Current page

163

Current page

164

Current page

165

Current page

166

Current page

167

Current page

168

Current page

169

Current page

170

Current page

171

Current page

172

Current page

173

Current page

174

Current page

175

Current page

176

Mechanical valve closure has many benefits – and it helped long-time proponent Ducati win the MotoGP championship
By Keith Howard

In most internal combustion engines the valves are opened by direct action of the camshaft lobe but closed by means of a spring. This works fine at low revs, but as revs increase, resonant ‘surge’ effects within a metal spring can play havoc with valve operation and reliability. This was why Renault began the trend to today’s ubiquitous use of pneumatic valve springs in Formula 1 – to break through the circa 14,000rpm rev barrier imposed even by titanium alternatives.

But there is another way to obviate valve spring deficiencies: do away with springs altogether and use a desmodromic system instead, in which the valve is both opened and closed by the camshaft. One conventionally shaped cam lobe pushes on the valve stem, via a follower, to open the valve, while a second D-shaped cam alongside pushes a second follower which pulls on the stem head to close it, usually via a forked end.

Desmodromic valve systems were briefly employed in early Grand Prix car engines (1914 Delage and Schneider, 1922 Rolland-Pilain), and enjoyed a short resurgence in the 1950s in the Mercedes W196 GP car – a development that encouraged others to investigate the system. But in only one place did that interest take root and carry through for the past 50 years – in Ducati road and racing motorcycles, pioneered by Ing Fabio Taglioni.

Shortly before Ducati Corse’s historic day at this year’s Japanese MotoGP race in late September – where Casey Stoner secured the Italian squad’s first MotoGP world championship and team-mate Loris Capirossi beat the might of the Japanese motorcycle industry on home territory for the third year in a row – I spoke to Ducati Corse director Filippo Preziosi about the desmodromic system, its development and why it still enjoys technical advantages.

“In the beginning the desmodromic system was developed because the spring materials of the time were not so good, so it was impossible to reach high revs. Now we are in a different situation in which you can achieve extremely good performance in terms of flow dynamics, and hence power, with pneumatic valve springs. But a pneumatic valve system is complex – it needs a reservoir, which is especially a problem for a motorcycle where the space is very tight. And a pneumatic valve system is also very expensive in terms of power losses compared with the desmo system, especially when you are running at low and medium revs.

“When you design a valve spring system, whether it uses pneumatic or metal springs, you have to put in enough spring force to allow it to work well at peak revs. Friction is related to this spring force, so when you are riding through a corner at 9000rpm rather than 19,000rpm there are big friction losses. The desmodromic system is better because it only generates the inertial force necessary to move the valve at that engine speed, so its friction is less.

“This is a great advantage in a championship where fuel consumption is so important because of the 21-litre fuel capacity limit. Also, if you use racing to develop your street bikes, it is no good using pneumatic valves, because they are not suitable for production machines. We can develop the desmo system in racing and our customers reap the advantage in their street bikes.

“Maximum engine revs are not limited by the desmodromic system but by the need to achieve a good balance between performance and fuel consumption. This season we are achieving something over 200bhp [from 800cc] and 19,000rpm, so we achieve similar engine speeds to F1 units – without using pneumatic valves.

“In the past we have had to detune engines to make the bike more rideable, or because the rider asked for it. Traction control helps a little but whatever we are able to do technically, the rider makes the biggest difference. In F1 you might get the two McLarens on the front row, the two Ferraris on the second row. So it’s clear the car plays a huge part in who wins the title. In MotoGP this year Casey Stoner had good tyres and a good bike, of course, but it was his excellent riding which won him the title.

“We suffered many problems of reliability with the desmo system during the early 1990s because we had designed it without having considered its dynamics. The behaviour of the desmodromic system seems very straightforward when you move it slowly, but it becomes extremely complex when it is running at very high revs. So we began two projects: experimental measurement of the system to determine the position, speed and the acceleration of each part – that’s easy to say but is difficult to do – and the mathematical modelling of the system to know better what happens in the valve mechanism when it is running at high speed. We have developed both of these approaches down the years, the experimental side and the calculations, so that now we feel that we understand the system very well and it is easy to manage.

“We found there were various things we could improve: materials, surface treatments, things like that. We could also improve the design to increase each part’s strength and stiffness. And we could properly design the cam lift.

“But there are small improvements still to be made in every part of the engine, including the desmodromic system. We have to reduce the power losses as much as we can, even though the desmo has an advantage in this respect.”

Fifty-one years on – still more to come.

You may also like

Related products