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Mildbrid Engine Offers Big CO 2 Emissions Reductions by John Mortimer Hybrid electric vehicles need not be as complex as current designs suggest. The IPA Mildbrid from Integral Powertrain in the UK offers a 25 percent reduction in CO2 emissions over a present-day direct injection (DI) diesel powertrain, yet it uses state-ofthe art technology.
Figure 1b: Cylinder head design torque/ speed
The diesel engine is now much improved but, due to theoretical thermodynamic and mechanical constraints and tougher emissions regulations, further significant efficiency improvements will be hard to achieve. But Integral Powertrain (IP) of Milton Keynes, UK, took a unique analytical approach to powertrain and vehicle modelling to investigate the gasoline engine as an alternative to the diesel as a low CO2 emissions power source. The result is a mild hybrid (‘mildbrid’) with the main internal components of the internal combustion (IC) engine shown in Figure 1a. The design of the cylinder head is illustrated in Figure 1b.
Today´s Technology The ‘mildbrid’ comprises a 20 kW Integrated Starter Alternator (ISA) and a 90 kW four-cylinder turbocharged gasoline IC engine to give the equivalent driving performance of a current 2-litre gasoline engine. The proposed powertrain delivers a 25% cut in CO2 emissions (EU Combined Cycle) compared to a state-ofthe-art DI diesel of equivalent performance. The cost penalty is only $600-$800 (see Figure 2), while the simple technology used would let manufacturers introduce the powertrain sooner and at less risk than more radical
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solutions to the CO2 challenge. A feature is the unique dual-mode Atkinson cycle for low speeds and un-wastegated turbocharging for high speeds. In order to make the proposition attractive, it was assumed that vehicle performance, driveability and NVH are comparable or better than the benchmark family car. Targets are: 0-50 km/h 0-100 km/h 80-120 km/h in top gear NVH
Fuel consumption (combined cycle) CO2
< 4.5 sec. < 11 sec. < 14 sec. Better than benchmark 2.0 litre gasoline engine; unotrusive starting < 5 l/100 km < 130 g/km
Particular areas for optimisation included: mass, engine operating cycle, torque curve shape, engine warm-up, cost and friction.
Four-Stage Approach A four-stage approach identified the optimum concept and confirmed its likely real-world performance and economy: Stage 1: Technology brainstorming (limited to production-ready concepts); Stage 2: Confirm the impact of individual technologies and complete powertrain concepts;
Figure 1a: The I4 gasoline engine uses state-of-the-art technology
Stage 3: Determine the most promising concept and complete a thorough analysis of its capabilities Stage 4: Confirm the in-vehicle performance and economy of the concept. IP’s engine modelling structure is shown in Figure 3. Built up from a number of modules, it is linked to a full-vehicle model. Fuel consumption per revolution is normalised against engine capacity to produce the output shown in Figure 4. The approach correlates with a range of gasoline power units and is largely independent of engine speed. In the range of speeds and loads consistent with the European Union (EU) driving AutoTechnology 6/ 2001
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Figure 2: The cost-penalty of the ‘Mildbrid’ is some $600-800
cycle, the relationship between normalized fuel flow and load is linear. This simple relationship can then be used to compare different technologies and calculate their effect on the load-independent fuel and the load-dependent fuel. Total drive cycle consumption can then be predicted from the total number of engine revolutions and the total work required to complete the drive cycle. Points on the graph indicate real test data from various engines. Using the data generated, plots in Figure 5 illustrate well how a hybrid powertrain reduces fuel consumption on the typical city driving cycle. The line can be lowered by downsizing the IC engine. For example, at very low vehicle speeds, the use of electriconly propulsion offers a significant benefit due to its minimal low-speed frictional losses. The steeper marginal efficiency of the electric drive system suggests that there is a point at which it is more efficient to switch to IC power. Using these methods produces a promising powertrain concept featuring: • A mild hybrid 20 kW electric machine • Auto-shift manual five-speed transmission • Homogeneous direct fuel injection (DFI)
• Turbocharged high-compression four-cylinder arrangement • Inlet cam with variable cam phasing • Partial Atkinson cycle The concept drastically cuts the fuel used to overcome friction and pumping losses of a typical gasoline powertrain and addresses the concerns of cost and weight. Critical areas were subjected to a detailed analysis to confirm viability.
Removing the low-speed cycle allows the valve train to be re-optimised using low-mass, low-cost direct-acting tappets optimised for low spring loads. The innovative air inlet completely eliminates the intake manifold by combining it with the charge cooler and cam cover. The geometry was optimised for high-speed power delivery to
Figure 4: Normalised fuel flow shown as a function of bmep
Detailed Analysis For example, the crankshaft train, together with the connecting rods, was downsized and optimised for a 1.1-litre engine capacity. This reduces the train’s mass and friction. Bearing oil consumption is also minimised, benefiting oil pump size and power demand. Careful design of cylinder head bolt load paths to minimise hot-clamped bore distortion requires long head bolts, anchored deep into the lower skirt of the block. Thermal bore distortion was optimised to produce very round bores within a range of engine temperatures, allowing low-friction piston ring packs. With no need for high oil pressure during idling, the oil pump can be further downsized.
Figure 3: Various models were used to produce the final design
complement the turbocharger. The turbocharger too is also novel and eliminates the wastegate, thus reducing cost and weight and maximising thermal efficiency. Normally, this would create undesirable torque characteristics, but here this is compensated for by the 20 kW electric machine (see Figure 6).
Figure 6: The integral electric machine and IC engine give improved torque/speed.
Figure 5: A hybrid can reduce fuel consumption in a city driving cycle
AutoTechnology 6/ 2001
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