UWE Bristol Engineering showcase 2015

Charles Winstone
Course Motorsport Engineering
Project Supervisor
Dr Chango Yang
Automotive Ignition Control
2 Stroke Engines
Unfortunately the traditional 2-stroke SI engine has neither been efficient or environmentally friendly suffering with high hydrocarbon
emissions and poor fuel economy. In recent years there have been many developments in this area solving many of the problems
mentioned. The reason for this is that once these problems are solved the 2 stroke engine has major potential thanks to a much higher
power to weight ratio and great improvement in mechanical efficiency over the 4 stroke engine. All of this allows vehicle engineers to
downsize the power train and improve performance in nearly every area.
Current Solutions
Currently there only exist s two realistic solutions to the problems facing two
stroke engines, these are Direct injection (DI) and homogenous charge
compression ignition (HCCI).
Direct Injection (DI)
Direct Injection is where an injector is placed directly into the combustion
chamber. The huge advantage is that the introduction of fuel can be timed as
such to be after the exhaust port has closed. As the exhaust port is closed
the usual problem of high HC emissions from the scavenging process is
completely removed. The impact on HC emissions is huge and in some case
reduces them below a representative 4 stroke engine. Another benefit is the
ability to move to a stratified charge cycle essentially allowing un-throttled
function while at the same time moving to a lean burn cycle giving a marked
increase in fuel efficiency.
Homogeneous Charge Compression Ignition (HCCI)
HCCI combustion is essentially a combination of spark ignited (SI)
combustion and diesel compression ignition (CI) combustion principles. The
engine draws in a pre mixed charge of fuel and air much like the SI engine
but then compresses it until auto ignition occurs as in a CI engine. The
combination of a diluted and premixed fuel and air mixture with multiple
ignition sites throughout the combustion chamber eliminates the high
combustion temperature zones and prevents the production of soot
particles, hence producing ultra low NOx and particulate emissions . Fuel
efficiency is also increased on average to around 30% better than the SI
equivalent from lean running.
In reality HCCI engines combine the best of both SI and CI engines with very
little downsides and hence that is the route taken in this project.
Project Solution
The project focuses on solving the main issue with HCCI engines. As there is
no spark plug and the fuel is pre mixed, controlling the combustion timing
requires a new control system. The only realistic way of controlling the
combustion timing is through the control of in cylinder pressure while
prediction the autoigntion timing, bringing the two together to control the
timing of the combustion.
Prediction of Autoignition
The prediction of auto ignition is possible as long as the pressure,
temperature and concentration of the gases are known. Unfortunately the
collection of these parameters is not viable for an internal combustion
engine. Empirical testing solves this issue by giving a relationship with inlet
manifold temperature and pressure vs inside the combustion chamber so
they can be predicted good enough. The actual prediction is controlled via a
modified for of the Livengood Wu equation which is used in SI engines to
predict knock. The output is set to out desired ignition timing and a value of
EGR is now known to produce this.CFD was also performed to validate these
results.
Exhaust Gas Recirculation (EGR)
Internal EGR is a method of directly reducing the amount of gasses that
escape through the exhaust port. As a consequence they are trapped inside
the combustion chamber waiting for the fresh intake charge, once the fresh
charge meets the trapped exhaust gases where the burnt gases heat up the
fresh charge to the required autoignition temperature. The operation of this
can be made to be very simple, implemented in various studies by installing
a butterfly valve in the exhaust manifold, which is what this project has
done. The final step was to calibrate the designed EGR valve to match the
predicted values so that the engine has effective control of HCCI combustion.
Project summary
This project is investigating the control of
ignition in a 2-stroke homogeneous charge
compression ignition (HCCI) engine.
Much research was taken to look into the
current technology, reasons for and theory
behind the phenomenon of HCCI combustion.
further to this the project then details the
application and design of such a system for
the control of a current 2 stroke spark ignited
engine.
Project Objectives
The main aim of the project is to design a
control system for the combustion events of a
2 stroke HCCI engine.
Project Conclusion
This report has demonstrated the application
of HCCI combustion to a small 2 stroke
engine , the benefits of running this cycle and
designed a system to convert a current SI
engine to HCCI.