J. G. Hawley

Dynamic Behaviour of a High Speed Direct Injection Diesel Engine

Many Diesel engine development programs concentrate almost exclusively on steady state investigations to benchmark an engines performance. In reality, the inter-action of an engines sub-systems under transient evaluation is very different from that evident during steady state evaluation. The transient operation of a complete engine system is complex, and collecting test data is very demanding, requiring sophisticated facilities for both control and measurement. This paper highlights the essential characteristics of a Diesel engine when undertaking testbed transient manouevres. Results from simple transient sequences typical of on-road operation are presented. The tests demonstrate how transient behaviour of the engine deviates greatly from the steady state optimum settings used to control the engine. The operation of the EGR system and its interaction with other sub-systems, in particular VGT, has a significant effect on emissions, fuel consumption and driveability, highlighting the need for dynamic optimisation as an integrated system.

Oxidative Stability of Biodiesel Fuel

Biodiesel, the fatty acid alkyl esters derived from vegetable oils, animal fats, or waste cooking oils, is an alternative to diesel fuel. One of the major technical issues with the use of biodiesel is its susceptibility to oxidation. Oxidation of biodiesel is a complex process which involves a number of mechanisms producing an array of chemical components such as aldehydes, acids, ketones, and oligomeric compounds. These components in turn increase the viscosity and deposits in the fuel beyond acceptable levels. A variety of factors affect the level of these decomposition products as well as the rate of formation and decay. These factors include the temperature, presence of light, catalytic metals in the fuel system, sump oil, or storage containers, type of biodiesel, fatty acid profile, blend level, other contaminants, and presence of antioxidants. This paper examines the relevant factors influencing the biodiesel oxidative stability, the methods used to analyse and test biodiesel oxidation, as well as the effect that oxidation has on the fuels properties.

Systems Approach to the Improvement of Engine Warm-Up Behaviour

Modifications to the coolant and oil circuits of a modern production 2.4 l diesel engine have been made in an attempt to promote oil warm-up to reduce fuel consumption. The new system used oil to cool exhaust gas recirculation (EGR) gases and incorporates a number of coolant flow control valves to reduce heat loss during warm-up. The engine was run over cold-start New European Drive Cycles with various flow strategies as a screening exercise to understand the behaviour of the system. Fuel consumption benefits of up to 4 per cent were observed, but these were accompanied by 3 per cent increases in nitrogen oxide (NO x ) emissions. Detailed analysis of the coolant flows and temperatures showed that, when throttling the flow, the mass of coolant in the degas bottle and radiator could be isolated from the system during warm-up, essentially reducing the thermal inertia. Heat transfer directly to the oil from the EGR gases rather than via the coolant allowed more heat to be put into the oil, with engine oil supply temperatures up to 6 °C hotter; however, it was not possible to verify that the oil was hotter at the bearings, valve train, and cylinder liner. The engine strategy was seen to react to the faster warm-up and to retard injection timing, reducing NO x but also compromising overall fuel consumption benefits. Further tests were conducted with various injection timings to establish a NO x —fuel consumption trade-off to demonstrate further benefits when the engine strategy is included in the operation of novel thermal management systems.

Review of the systems analysis of interactions between the thermal, lubricant, and combustion processes of diesel engines

Abstract A review of technologies surrounding the thermal management system of the modern diesel engine with increased attention on fuel consumption is presented. A system-based approach has been adopted, looking at the interaction with other key systems. Previous innovation has aimed at reducing the power consumption of the cooling system or incorporating different cooling strategies and improving the engine warm-up rate for improved fuel consumption by higher operating temperatures. Electrical pumps can operate independently of the engine speed, and precision cooling and nucleate boiling have improved the heat transfer within the engine, reducing coolant flow requirements by 90 per cent. Improved warm-up rates have been demonstrated by using reduced thermal inertia or energy recovery systems either simulated on the test rig or through heat exchangers with exhaust gases. The resultant reduction in the fuel consumption is a result of various effects of the temperature on both the lubricating system and the combustion process. Despite difficulties in accurately measuring the engine friction, studies suggest that an increase in the engine temperature from 50°C to 80°C reduces the engine friction by 44 per cent because of 67 per cent lower oil viscosity. Simultaneous reduction in the emissions of nitogen oxides (NO x ) and the fuel consumption of 13.5 per cent and 0.7 per cent respectively have been achieved by including the engine thermal system in the calibration procedure. However, in-cylinder data needs to be studied to understand fully the mechanisms involved. Hotter engine temperatures reduce ignition delay, making combustion occur earlier in the cycle, which has a positive effect on the fuel consumption but a negative effect on the NO x emissions. Engine thermal management requires a system-based approach if the effects are to be fully understood but offers potential as an additional parameter in engine calibration.

Experimental and modelling aspects of flow boiling heat transfer for application to internal combustion engines

Abstract A detailed programme of work has been undertaken to quantify the suitability of predictive methods for accurate determination of the levels of boiling heat transfer within an internal combustion (IC) engine cooling gallery simulator. An extensive array of experimental data has been obtained as the basis for the predictive validation. Working on the principle of superposition, the convective component of heat transfer has been represented by the established Dittus-Boelter correlation which has been extensively modified to account for developing boundary layers, surface roughness and nearwall viscous effects. The boiling component has been represented by the Chen model, modified for binary fluids and subcooling. For the IC engine cooling application it is concluded that the application of the Chen approach must be complemented by a convective heat transfer model that accurately represents the complex thermo-fluid situation being experienced within a developing flow.

A fully analytical treatment of heat release in diesel engines

Abstract The paper details a wholly analytical approach to heat release modelling. The approach is based on the diffusion combustion process which is specific to diesel engines operating under very high injection pressures. The combustion is subsequently controlled mainly by two items, the instantaneous fuel mass present in the cylinder charge and the local density of turbulent kinetic energy. Analytical solutions are developed for each. An extensive test series was undertaken, covering the limiting torque curve (LTC) between 1250 and 4000 r/min and utilizing a high-pressure common rail diesel engine to generate the necessary validation data. The validation exercise has shown the flexibility of this heat release modelling approach to be extremely accurate and suitable for further development.

The impact of biodiesel blend ratio on vehicle performance and emissions

Abstract Biodiesel is synthesized via the transesterification of triglycerides contained within vegetable, animal, or waste oils. First-generation biofuels are not the solution to global transport energy needs; however, biodiesel does have a role to play in reducing greenhouse gas emissions from the transport sector, so long as necessary production can be achieved in a sustainable manner without negative impact on plant and animal biodiversity. The biodiesel content within diesel sold to consumers is set to increase in the future, with implications on vehicle fuel consumption, emissions, and base engine durability. This study examines the effects of increasing the biodiesel blend ratio on the performance and emissions of a production vehicle equipped with a common-rail direct-injection diesel engine, evaluated on a chassis rolls dynamometer, at various ambient temperatures. Results obtained show that reductions in engine-out carbon monoxide and hydrocarbon emissions do not always translate to lower tailpipe emissions as reduced exhaust gas temperatures at higher blend ratios lead to reduced catalyst conversion efficiencies and higher total cycle emissions. Catalyst conversion efficiencies for carbon monoxide and hydrocarbons over the New European Drive Cycle (NEDC) are reduced by 9–19 per cent (depending on the ambient temperature) for a 50:50 blend (B50) compared with the petroleum diesel (B0) baseline. Increasing the blend ratio caused a linear decrease in the vehicles maximum tractive force. This reduction was of the order of 5 per cent for a B50 blend at low vehicle speeds and 6–10 per cent at higher speeds, which is greater than would be expected on the basis of the differences in calorific values. Over the NEDC, the fuel consumption was found to increase with increasing blend ratio.

Vehicle modal emissions measurement: techniques and issues

Abstract The measurement of vehicle modal emissions is technically challenging owing to the major issue of determining exhaust-gas mass flowrate and ensuring that it is synchronous with the corresponding ‘slug’ of gas to be measured. This is also extended to the simultaneous measurement of pre- and post-catalyst emissions to determine small passive NOx conversion efficiencies. Although only really evident for passive NOx conversion efficiencies where the magnitude of catalyst performance is low in comparison to HC and CO, a misalignment between these measuring points of between will cause the resulting NOx conversion efficiency to lie anywhere between 0 per cent and 20 per cent. Further alignment issues arise when the CO2 tracer method is used for determining exhaust-gas volume flowrates. The sensitivity of time-alignment along with techniques and associated issues concerned with modal gas-flow measurement is presented in this paper.

Transient Investigation of Two Variable Geometry Turbochargers for Passenger Vehicle Diesel Engines

ABSTRACT The use of variable geometry turbocharging (VGT) as anaid to performance enhancement has been the subject ofmuch interest for use in high-speed, light-duty automotivediesel applications in recent times (4). One of the keybenefits anticipated is the improved transient responsepossible with such a device over the conventional fixedgeometry turbine with wastegate. The transient responses of two different types of variablegeometry turbocharger have been investigated on adynamic engine test bed. To demonstrate the effect of theturbocharger on the entire system a series of stepchanges in engine load at constant engine speed werecarried out with the turbocharger and exhaust gas recir-culation (EGR) systems under the control of the enginemanagement microprocessor.Results are presented which compare the different per-formance and emissions characteristics of the devices.Some control issues are discussed with a view to improv-ing the transient response of both types. Of particularimportance is the interaction between the turbochargersystem and the EGR system.

Convective coolant heat transfer in internal combustion engines

Abstract Simple heat transfer correlations are known to underpredict the single-phase convective heat transfer coefficient when applied to internal combustion (IC) engine cooling passages. The reasons for such underprediction were investigated using a specially designed test rig which was operated under a wide variety of test conditions relevant to IC engine operation. Data from this rig study identified that undeveloped flow (fluid dynamically and thermally), surface roughness and fluid viscosity variation with temperature were the physical reasons responsible for the mismatch. Simple empirical heat transfer models have subsequently been extended to take account of these factors and are shown to give much improved correlation with rig data, and data from an engine study. The implications of this work for predicting engine heat transfer in a three-dimensional computational fluid dynamics environment are discussed.