SUPERIOR DOWNSIZING
5.1 Introduction
evolving emissions legislation and concerns for diminishing fuel reserves continue to prompt automotive manufacturers to seek alternative forms of engine operation. in recent years, european CO2 emissions targets have largely been met through increased diesel sales, rising above ~50% in some countries. However, the distillation of crude oil results in high proportions of both gasoline and diesel fuel. in terms of well-to-wheel energy expenditure and emissions, such diesel market penetration cannot continue indefinitely (1) and is not yet foreseen outside europe (2). in order to meet future global emissions targets in the short to medium term, it will be necessary to improve the fuel consumption of the gasoline engine.
A large proportion of automotive engine operation is spent at part-load and this is where fuel economy targets are currently most concerned. However, under light loads the breathing losses of the spark ignition (si) engine are usually at their highest levels. Downsizing has therefore become widely considered to be one promising method for improving the fuel consumption of the gasoline engine (3–5). The basic principle is to reduce the capacity of the engine and hence enforce a larger proportion of operation to higher loads.
As a result, the pumping losses of the engine can be significantly reduced for a given road load requirement. in order to compensate for the inherent power loss, some form of intake air pressure charging is usually required.
However, the associated rise in peak in-cylinder gas temperatures and pressures in a unit of high power density can result in an increased tendency to knock. The onset of such knocking combustion is generally accepted to be the result of excessive energy release rate via exothermic centres in the end-gas beyond the entraining edge of the flame (6). Such auto-ignition is often multiple-centred and may occur as one or more of three identified modes (namely deflagration, detonation and developing detonation). The developing detonation mode, which can be induced via preceding deflagration events, is believed to be the most damaging in the si engine (7).
Current methods to avoid knock involve retarding the spark and/or cooling the mixture with excess fuel, albeit with the penalty of increased fuel consumption and operation away from the fixed stoichiometric mixture strength required for efficient catalytic conversion. However, even under such conditions, in an engine of high energy inhalation rate it is usually still necessary to reduce the geometric compression ratio to avoid knock.
As a result, in recent years various workers have investigated the synergy between downsizing and direct fuel injection, the charge cooling effects of which facilitate increased compression ratio and improved thermal efficiency (8–12). However, such injection systems are not without their own challenges, including maintaining robust pollutant emissions control.
shown in Fig. 5.1 is a map of relative air-to-fuel ratio (l) for a typical turbocharged gasoline direct fuel injection engine. While the regime of fuel enrichment is normally at loads higher than those encountered during most
24 22 20 18 16 14 12 10 8 6 4
BMEP (bar)
1000 2000 3000 4000 5000 6000
Engine speed (rpm) 1.00 0.95 0.90
0.85 0.80
1.00 0.90
0.95
5.1 Relative air-to-fuel ratio (l) speed–load map for a typical turbocharged gasoline engine.
existing drive cycle assessments, it is possible that full load fuel economy targets and emissions legislation will emerge in future years. Furthermore, the more aggressive the downsizing approach, the more often the over-fuelling region will be encroached upon during ‘real world’ driving conditions.
Various investigations have therefore been made into alternatives to fuel enrichment at high output. Of these, the most promising methods studied in recent years have included cooled exhaust gas recirculation (eGr), lean boost (13, 14), indirect water injection (15) and so-called turbo-expansion (16), and it is the EGR approach that is the topic of this chapter.
The concept of using cooled eGr to improve gasoline high-load fuel consumption is not a new one. Brustle and Hemmerlein (17) studied the effects of cooled external eGr on the performance of a four-cylinder turbocharged engine operating under stoichiometric gasoline fuelled conditions. At high engine speed and load and a pre-turbine temperature limit of 1000°C, an increase in brake mean effective pressure (BMeP) of ~1 bar was obtained when recycling up to ~12% eGr. These workers postulated that use of eGr could allow small increases in compression ratio to be tolerated, with improved fuel economy across the entire operating map. Specific details of the eGr circuit performance and layout were not reported.
in later work, Grandin et al. (18) investigated the influence of cooled external eGr at high loads in a turbocharged four-cylinder 2.3-litre gasoline port fuel injection (PFi) unit. The eGr gases were taken from before the turbine, passed through a cooler and introduced to the inlet downstream of the compressor (post-intercooler). it was concluded that eGr dilution served to decrease the rate of mass burning at high speed and load, with reduced temperature rise during combustion and significant inhibition of knock.
Although the results presented were limited to 4000 rpm, these workers demonstrated the potential of eGr to replace fuel enrichment and vastly improve emissions of carbon monoxide (CO) and hydrocarbons (HC). later on (19), the technique was compared with excess air dilution at speeds and loads of up to 5000 rpm and 16.5 bar BMEP respectively. Although combustion stability was observed to deteriorate in each case, it was apparent that eGr facilitated the more stable burn and presented significant advantages in terms of reducing emissions of nitrous oxides (nOx). in an alternative but similar study in a turbocharged four-cylinder 2.0-litre unit, Duchaussoy et al. (20) concluded that introducing EGR provided significant benefits over excess air dilution in terms of transient turbocharged engine operation, with a smaller increase in compressor size required. Specific details of the pre- to-post compressor EGR feed ratio (with the flow divided via a three-way valve) were not provided. elsewhere, Diana et al. (21) also demonstrated the knock inhibition effects of eGr at moderate to high output in a PFi research engine, albeit without pressure charging and hence incurring significant power loss. Regardless, these workers confirmed the effects earlier postulated by
Brustle and Hemmerlein, with up to ~10% improvement in fuel economy achieved at moderate load with the geometric compression ratio increased from 10 to 13.
In more recent work, Alger and co-workers (22) studied the benefits of eGr when applied to a modern production turbocharged gasoline Di engine.
This study clearly demonstrated the potential of eGr across the entire excess fuel operating map, achieving reductions in CO2 of up to ~20% that were associated with reduced knocking tendency and hence lower exhaust gas temperatures. reductions in CO of up to ~95% were also recorded at high speed and load. elsewhere, Kapus et al. (23) have insinuated that the eGr technique should perhaps be combined with a water-cooled exhaust manifold to reduce exhaust gas temperatures to a level where diesel-like variable geometry turbochargers could be used for spark ignition applications.
in summary, partially due to strong synergy with the engine downsizing approach, the so-called ‘wide open throttle eGr’ (WOT-eGr) technique is now subject to significant research efforts, including those at MAHLE Powertrain (24, 25).