|
|
Page 5
Lotus Service Notes
Section EH Note that compromises are involved in the programming of inlet cam timing, since advancing the valve opening point also advances the valve closing point, when the ideal might be to advance the opening and retard the closing points. For any particular engine running conditions, the timing is adjusted to optimise either the valve opening point and overlap period, or the valve closing point, whichever provides the most benefit. The range of inlet cam timing available is from: Opening 33° BTDC, Closing 15° ABDC ) to; ) with standard (low speed) valve lift Opening 10° ATDC, Closing 58° ABDC ) or; Opening 58° BTDC, Closing 54° ABDC } to; } with high speed valve lift (see below) Opening 15° BTDC, Closing 97° ABDC } Variable Valve Lift Both inlet and exhaust camshafts are machined with two cams for each cylinder, a low lift cam and a high lift cam. Each low lift cam actuates, via a low friction roller, a rocker arm which connects with a pair of inlet or exhaust valves. The corresponding high lift cam actuates a spring loaded tappet housed within the rocker arm, and under low speed conditions, has no effect on valve operation due to the clearance between the bottom of the tappet and the rocker arm. When engine speed reaches 6,000 rpm at normal running temperature, the ECU operates a spool valve on the back of the cylinder head to close an oil return line, and raise oil pressure within the rocker pivot shaft and passages within each rocker. This increased oil pressure is sufficient to overcome the spring loading of a packer pin contained within each rocker arm, which is then forced between the bottom of the high speed cam tappet and the rocker arm. Each high lift cam then controls valve operation, with the rocker being lifted clear of the low speed cam. The higher valve lift for both inlet and exhaust valves in conjunction with the variable valve timing, provides greater efficiency and power output at high engine speeds. Standard (low speed) valve lift: - inlet; 7.25 mm - exhaust; 7.25 mm High speed valve lift: - inlet; 11.4 mm - exhaust; 10.0 mm Note that engine speed is limited to 6,000 rpm until normal running temperature has been attained. Illustrations overleaf............................... |