Analysis of Transient Operation of Turbo Charged Engines

Transient operation of turbocharged engines is mostly optimised in the light of quickness of response and the provision of the demanded torque. The time from demanded boosted torque to delivered torque above the maximum torque provided by the natural aspirated torque value is known as turbo-lag. This could reveal as an issue for small gasoline turbo-charged engines with a displacement of 1.0ltr or lower. These small types of engines are moving more and more in the focus for automobile applications. To provide the required power and torque, gasoline direct injection and turbo-charging are helpful in order to enable a reduction of fuel consumption by both de-throttled operation over a large area of operation and improved thermal efficiency among others achieved by maintaining an appropriate compression ratio. However, the application of a turbocharger especially on small engines drives also the back-pressure at the exhaust, in turn affecting the knock sensitivity of the combustion and thus impacting efficiency and power negatively. To achieve low levels of back-pressure even on small engines, the size of the turbocharger is from an efficiency point of view to be chosen of a rather large scale. In turn, a larger turbocharger increases the turbo-lag. Thus, an appropriate matching process of the turbocharger is a careful balance between efficiency, maximum torque and transient response, and in the light of stringent downsizing, this thematic deserves closer attention. The paper presents a new approach to characterise the transient operation and response behaviour of turbochargers while applying a new combination of established and novel approaches. Firstly, the interaction between vehicle parameter and shifted gear during acceleration is presented. The difference between the development of the torque delivered during transient operation compared to the steady state torque curve gathered from a dynamometer test of the engine is elaborated. Secondly, the turbo-lag is characterised by using the acceleration and corresponding jerk of the vehicle during an acceleration event. Based on these pre-requisites, the transient response of a C-car vehicle equipped with a small-engine is shown comparatively with a vehicle propelled by either a port-fuel injected natural aspirated gasoline engine or a turbo-charged diesel engine. Finally, a novel method to measure and analyse the turbo-lag using noise signals is presented. The advantage of this approach is that driveability and noise measurements can be done in parallel, leading to a reduction in set-up time and, moreover, allowing a more in-depth analysis of the interaction of transient driveability and attributes. Furthermore, the relation between subjective perception and objective measured data is demonstrated.