The invention solves the problem that the quantity of unburnt gas is increased if a vehicle speed is limited by thinned-out ignition, CO, HC gas is increased and a load of the catalyst is increased, and the service life of the catalyst is considerably reduced. (a) shows control sections. A first vehicle speed threshold and a second vehicle speed threshold which is higher than the first one are set, and normal control is performed to the first vehicle speed threshold, lean control is performed between the first vehicle speed threshold and the second vehicle speed threshold, and the lean control and the ignition control (the thinned-out ignition) are performed above the second vehicle speed threshold. (b) shows the opening/closing operation of a solenoid valve, and (c) shows ignition control, and more specifically, the presence/absence of the thinned-out ignition. The vehicle speed is gradually reduced by the lean control in a first stage, and the vehicle speed is reliably reduced by the ignition control in a second stage. In the lean control, combustion is continued, the quantity of the unburnt gas is reduced, and an emission level can be increased. |