Fig. 2a and b shows the progress of rotating speed and EGR
valve opening during cold and warm start, respectively. As shown,
the starting transient process until reaching the idling speed occurred
in 1 s regardless of the fuel used and the thermal conditions
of the starting. Moreover, independently of the fuel tested, the idling
speed corresponding to the warm start was 800 rpm, approximately
100 rpm less than at cold start, since less injected fuel is
needed to achieve stationary conditions. It should also be noted
that during cold start the idle speed was slightly higher when using
GTL as fuel, because the relative fuel–air ratio registered with this
fuel was slightly higher, as shown in Fig. 3a.