The effects of basic engine operating parameters on particle emissions from a hybrid city bus were examined under urban driving conditions involving both constant speed and frequent acceleration. To perform the examinations, real-time data for the parameters and solid Particle Number (PN) emissions were synchronously collected. The driving conditions were specifically chosen because, depending on the driving conditions, PN emissions from a city bus may change by an order of magnitude. A PN measurement system highly sensitive to engine transients and background emissions was specifically chosen, because the particle emission levels typically found in modern diesel engines are relatively low. The measurements indicated that while operating over a flat route, the PN emissions of the bus were maximized at full-load accelerations and minimized at a constant speed of 70 km/h, which is the maximum speed. Because the combustion process in a diesel engine is relatively stable under constant speed driving conditions, the particle formation was Minimized. During acceleration, however, the particle formation is accelerated due to well-known turbo-lag phenomena that limit the intake air flow rate. For these reasons, there is no simple linear correlation between the PN concentration and the engine operating parameters for driving conditions that involve both acceleration and constant speed driving. However, it was observed that there is a strong correlation between the cumulative PN emissions and cumulative brake engine energy (BEE). The R-2 values of the linear regression lines were almost 1.0 when the acceleration and constant speed operating conditions of the bus were separately evaluated. Because a city bus operates under real-world conditions involving both accelerating and constant speed driving conditions, a real world PN emission factor was suggested as a function of the maximum, minimum PN emissions (#/kW h) of the bus and a traffic intensity factor. (C) 2014 Elsevier Ltd. All rights reserved.