Effects of transient diesel engine operation on its cyclic heat transfer: an experimental assessment

Abstract

This paper presents results from the analysis of experimental data with the aim of investigating the cyclic, instantaneous heat transfer phenomena occurring in both the cylinder head and exhaust manifold wall surfaces of a direct injection, air-cooled diesel engine, operating under transient events (long-term), viz. during a sudden change in engine speed and/or load, rather than the usual steady state. The experimental installation allowed both for long- and short-term signal types to be recorded on a common time reference base during the transient event. Processing of experimental data was accomplished using a modified version of one-dimensional heat conduction theory with Fourier analysis, capable of catering for the special characteristics of transient engine operation. Based on this model, the evolution of local surface heat flux during a transient event was calculated. Two engine transient events are examined, which present a key difference in the way the load and speed changes are imposed on each one of them. It is revealed that in the case of a ‘ramp’ transient the amplitude of wall surface temperature and heat flux variation in the combustion chamber during the first 20 engine cycles was increased by almost three times more than the corresponding ones during steady state operation. At the same time, temporal gradients of cylinder pressure and wall surface temperature are also sharply increased in the combustion chamber and exhaust manifold surfaces. The variation of cylinder pressure, surface temperature, and heat flux is altered significantly during the transient event, with their peak values either advanced or retarded compared with those for the corresponding steady state operation. The previous observations are moderated or even vanished in the case of a ‘slow pace’ transient variation, revealing the significant influence of engine speed and load triggered by the external command, on the response of heat transfer variables. During a ramp transient, the amplitude of wall temperature swing is increased significantly even at a depth of a few millimetres below the surface of the combustion chamber, thereby influencing the dissipation of heat towards its external layers. Under these circumstances, the depth of penetration for temperature oscillations inside the metal volume is also significantly increased.

Development of combustion instability and noise during starting of a truck turbocharged diesel engine

“…As the engine speed gradually increased, the rack moved progressively to a lower fuel supply position until it ultimately assumed its final steady-state value after the engine had reached its idling, self-sustained speed. It is worthwhile mentioning that, even after the engine speed had stabilised, the whole phenomenon continued to develop from the thermal point of view, since a much longer duration is required for the stabilisation of exhaust gas, coolant and lubricating oil temperatures, as well as for their cylinder and exhaust manifold wall counterparts owing to their high thermal inertia (Rakopoulos and Giakoumis, 2009;Rakopoulos and Mavropoulos, 2009). This thermal transition lasted for at least a few minutes; a relatively long period compared with the duration of the actual starting event.…”

Experimental Study of Transient Nitric Oxide, Smoke, and Combustion Noise Emissions during Acceleration of an Automotive Turbocharged Diesel Engine

“…The main mechanism behind the increase in combustion noise radiation during transients lies in the operating principles of a transient event. During the first cycles after a speed (or load) increase, the injected fuel quantity has already increased substan- tially cooling down the charge-air temperature; however, the cylinder wall temperature is still low (up to 100 uC lower than the corresponding steady-state conditions) as the thermal transient proceeds at a much slower rate due to the cylinder wall thermal inertia [42]. The combination of increased fuelling with the still low cylinder wall temperature and 'cooled' charge-air temperature results in longer ignition delay, hence more intense premixed combustion periods leading to steeper cylinder pressure gradients and, consequently, higher combustion noise levels [2,26].…”

Analysis and evaluation of the thermal shock phenomena in the in-cylinder surfaces of a DI diesel engine during its transient operation

“…Especially during recent years, engine transient heat transfer phenomena have been investigated by the present [37,38] and also other research groups [39]. Bohac et al [40] have presented a global model characterizing the thermal behaviour of internal combustion engines.…”

“…For many years, owing to their high importance, heat transfer processes from both high-and lowfrequency categories have been a subject of interest for a lot of research groups and institutes related to internal combustion engines research and development. Especially during recent years, engine transient heat transfer phenomena have been investigated by the present [37,38] and also other research groups [39]. Bohac et al [40] have presented a global model characterizing the thermal behaviour of internal combustion engines.…”

“…Using a simulation code of the thermodynamic cycle of the engine during transient operation, they have examined the effect of various insulation schemes in transient engine operation on a cycle basis. In addition, they have investigated and presented the significant influence and interaction that exists between the long-term non-periodic variation of combustion chamber temperatures caused during the transient engine operation, and the short-term cyclic fluctuations of surface temperatures and heat fluxes [37,38], providing a detailed description of the physical mechanism that governs them.…”