Abstract:
This work proposes to match the engine characteristics to the requirements of
the Continuously Variable Transmission [CVT] powertrain. The normal process is to
pair the transmission to the engine and modify its calibration without considering the
full potential to modify the engine. On the one hand continuously variable transmissions
offer the possibility to operate the engine closer to its best efficiency. They benefit from
the high versatility of the effective speed ratio between the wheel and the engine to
match a driver requested power. On the other hand, this concept demands slightly
different qualities from the gasoline or diesel engine. For instance, a torque margin is
necessary in most cases to allow for engine speed controllability and transients often
involve speed and torque together. The necessity for an appropriate engine matching
approach to the CVT powertrain is justified in this thesis and supported by a survey of
the current engineering trends with particular emphasis on CVT prospects. The trends
towards a more integrated powertrain control system are highlighted, as well as the
requirements on the engine behaviour itself.
Two separate research axes are taken to investigate low Brake Specific Fuel
Consumption [BSFC] in the low speed region and torque transient respectively for a
large V8 gasoline engine and a turbocharged diesel V6 engine. This work is based on
suitable simulation environments established for both engines in the powertrain. The
modelling exercises are aimed at supplying appropriate models that can be validated
against experimental data. The simulation platforms developed then allow the
investigation of CVT powertrain biased engine characteristics.
The V8 engine model in particular benefited from engine and vehicle
dynamometer data to validate the model behaviour and the accuracy of the prediction. It benefited from the parallel work conducted on the Electrically Assisted Infinitely
Variable Transmission [EASIVT] project in Cranfield University. The EASIVT vehicle
is a parallel mild hybrid aimed at demonstrating the combined fuel economy benefits of
a CVT technology and hybridisation. From the CVT powertrain requirements for fuel
economy, BSFC operation can be further promoted in the low speed region if Noise
Vibration and Harshness [NVH] counter-measures are developed. A study of the
combustion torque oscillations at the crankshaft led to the elaboration of an Active
Vibration Control [AVC] strategy for the hybrid Integrated Motor Generator [IMG].
Successful implementation of the strategy in both simulation and in-vehicle helped
quantify the benefits and short comings of engine operation for best fuel economy. The
development in parallel of the hybrid control functions for torque assist and regenerative
braking made it possible to implement the low speed AVC in the vehicle without a
driveability penalty.
The V6 TDI model yielded a realistic and representative simulation for the
transient torque response improvement research to be undertaken. For that purpose, the
model was tuned against full-load data and the air path control sub-systems were
designed and calibrated similarly to a real application. The model was able to highlight
the turbocharger lag issue associated with a large combined speed and torque transient
inevitable in the fuel economy biased CVT powertrain. This study proposes a Manifold
Air Injection [MAI] system in the intake of the engine to help breathing when the VGT
operating conditions cannot be shifted rapidly enough for a manoeuvre. The system
design constraints were analysed and a suitable strategy was elaborated and calibrated.
A sensitivity analysis was also conducted to demonstrate the influence of the MAI
design and control variables on the engine performance in the CVT powertrain
In conclusion, the benefits of the engine characteristic matching were
highlighted in both cases. A review of the work achieved is available in the last chapter,
including prospects for further improvements and investigations. The ideal engine
characteristics for gasoline and diesel engine technologies integrated in a CVT
powertrain are derived from the experience gathered in the research and the results
obtained from the tests in low speed operation and transient torque control respectively for the gasoline and the diesel engines. The engine characteristics can be altered toward
a better match with a CVT by the use of specific hardware and control strategy.
This work recommends that a direct injected, variable valve actuated gasoline
engine provides the ideal starting point for low fuel consumption powertrain. When
integrated within a mild hybrid CVT powertrain, the full benefits are obtained with the
use of low speed operation and AVC. If no electrical machine is available to torque
assist the engine, then existing supercharging concepts for a downsized engine can be
applied.
Diesel engines can also be downsized because of their high torque density.
Increased turbocharging boost levels allow steady state torque levels to be maintained in
the downsizing process. The CVT powertrain can optimise the fuel consumption and
emission levels by appropriate selection of the engine steady state operating points. The
torque response lag then becomes critical for the CVT to control the engine speed. This
can be improved by the use of Manifold air Injection to assist the turbocharger.