Despite the growing trend towards electrification of the drive train, internal combustion engines still remain the predominant form of propulsion for vehicles, and are set to do so for many years to come.
This creates some pretty daunting challenges for the automotive industry:
• Internal Combustion engines need to comply with ever tightening emissions standards
• Fuel consumption has to be reduced to meet the growing demand for economical cars
• Customers’ performance expectations have to be fulfilled
Facing these constraints the industry is rapidly moving towards engines with a lower cubic capacity but greater specific power. This accomplishment is largely enabled through turbo charging, as it’s the only way to downsize an engine, which is an essential development if fuel consumption is to be reduced, without significantly reducing power.
According to August Hofbauer, Continental’s North American director of engineering and sales for turbo systems, V-8s will be replaced by V-6s and Inline-4s; which in turn will be replaced by a new generation of potent 3-cyl. engines.
Furthermore, Continental predicts half of new light-vehicle petrol engines in the U.S will be turbocharged by 2025. While Honeywell predicts that globally, about 40% of petrol engines should be turbocharged by 2017.
Ford is one of the manufacturers that has invested heavily in downsizing and turbo charging technologies, and in 2013 became the first manufacturer to produce a sub 100g/km (Co2 emissions) segment C petrol vehicle in Europe.
Fitted to the Focus, the engine is a recalibrated turbocharged 999cc three-cylinder that emits 99g/km CO2 emissions; which is similar to that achieved by diesels but at a lower cost.
Diesels first crossed the 100g/km threshold in 2009, with petrol engines set to follow a similar pattern: When Ford launched the 999cc three-cylinder in the Focus in 2012, the 74kW rating emitted 109g/km when offered with a stop-start system and a five-speed manual transmission.
However, for extreme downsizing, even the most advanced single-turbo systems may not be capable of producing enough boost pressure, or maintaining acceptable transient response.
The recently completed collaborative project ‘Ultra Boost for Economy’ (or ‘Ultraboost’), initiated and funded by the UK Technology Strategy Board, tested several technologies surrounding downsizing and forced induction.
The limitations of turbo charging when applied to severe downsizing.
The project’s objective was to explore the feasibility of achieving drive-cycle fuel economy improvements, in the region of 35%, through the aggressive ‘downsizing’ of a large capacity naturally-aspirated engine; without the use of hybridisation. The project partners were Jaguar Land Rover (JLR), Lotus, GE Precision, CD-adapco, Shell, the University of Bath, Imperial College London and the University of Leeds.
The general objective of achieving the same torque curve of the naturally-aspirated version of the 5.0 litre Land Rover AJ133 V8 engine was an inherent part of the project, along with the ability to provide driveability comparable with the Land Rover V6 diesel engine.
An outline specification of a 2.0 litre four cylinder pressure charged DISI engine, representing a 60% downsizing which would need to operate at up to 32 bar Brake Mean Effective Pressure (BMEP), was decided upon at the outset.
This downsizing achieved significant reductions in drive-cycle and real-world fuel consumption through operating the engine at a higher load (for any given road condition) and a reduction in friction through, in this case, a significant reduction in the number of engine cylinders.
For a given rating, smaller engines run at higher load and are therefore more efficient but have a greater tendency to knock. Nevertheless, despite the Ultraboost engine running at more than 35bar BMEP pre-ignition was rarely a problem during development, according to JLR’s principal powertrain engineer, Dr James Turner; adding that the limit to extreme downsizing appeared to be boosting technology, not the combustion system itself.
However, an extremely challenging target for the project was to match the torque curve of the original V8 through achieving 25 bar BMEP at 1000 rpm, where preignition and other abnormal combustion phenomena would normally be expected to be a severe limitation. Using cooled EGR as well, the engine easily produced the initial high-load torque targets with extremely good specific fuel economy figures and virtually no preignition.
In order to achieve these results a Honeywell turbocharger was selected as a low-pressure stage and an Eaton supercharger in series, as the high pressure stage. The supercharger is fully declutched above 3,000 rpm since it is being driven at very high speed to provide the necessary boost for driveability and response at lower engine speeds.
Although the Ultraboost project delivered a 15% improvement in fuel efficiency with a peak output of 550Nm it was also clear that further potential could be exploited using more efficient turbos and superchargers.
Technology offering improved efficiencies.
In an effort to further improve turbo efficiency Honeywell Turbo Technologies is working on ultra-low inertia turbos for downsized petrol engines. Featuring axial flow turbines and dual-sided compressors, these offer better response than existing twin-scrolls.
While most turbos in automotive applications feature radial turbines, in which the air flows in vertically and exits horizontally, DualBoost turbos have the air flowing in horizontally and exiting in the same direction.
Employing zero-reaction aerodynamics, no nozzles and tall-blade design enables the axial turbines to attain higher specific speeds, allowing a smaller size for the same flow, thereby lowering inertia by up to 40 percent. In addition, the axial turbines are designed for good efficiency under pulsating flows found in inline 4 cylinders.
Adopting a system approach and applying innovations in both turbine and compressor stages, resulted in a smaller turbo that matches the aerodynamics of a larger component while delivering a 25 percent improvement in time to torque.
Honeywell’s project leader, Vit Houst, explains: “It’s a specially designed axial turbine. The advantage is mainly a reduction in the moment of inertia for any given flow when compared to a radial turbine. It’s natural that we paired it with a double-sided parallel compressor. This also utilizes a bearing designed specifically for this concept.”
According to Honeywell; despite the increasing popularity of boosted three-cylinders, the four-cylinder petrol engine market has the greatest growth potential over the next five years. These engines do not have the same inherently good exhaust pulse separation and require better, more efficient turbos as ratings continue to rise.
Although currently only found in Diesel applications, Honeywell have introduced Ball Bearing technology to passenger car turbos. The new “low-friction” turbo, which runs on two rows of ceramic balls, has much lower mechanical losses and hence higher turbo efficiency. The result is an improvement in transient response of between 20-70 percent dependant on oil temperature and a 2 percent improvement in fuel economy in standard test cycles.
Looking towards the future of downsizing and forced induction, Honeywell predicts that Variable Nozzle turbos will play an increasingly dominant role in TwoStage systems. Whilst at the same time ball bearing usage, providing significant benefits at lower turbocharger-shaft speeds and expansion ratios, is set to increase in Two Stage systems. Furthermore, ball bearings’ higher thrust-load capacity, lower oil flow requirements and superior gas sealing capabilities also bring inherent system-level benefits to these applications.
Two stage turbochager development.
While a single-stage turbocharger architecture is well adapted to light downsizing of up to 30% cubic capacity while keeping the same power output; for downsizing by 50%, going from a 3.5-L six-cylinder to a 1.8-L 4-cylinder for example, other solutions need to be adopted.
Recognising the benefits of two-stage turbocharging for downsized petrol engines, Ford is currently working on a boosting system with BorgWarner which enables a prototype 1.6-litre four-cylinder to produce 188kW and 360Nm.
Kai Kuhlbach, Ford powertrain R&D project manager explains the results: “Across the entire speed range very high charge pressures are achieved, with a very effective combustion and stoichiometric performance under full load. Even at high loads, good consumption is achieved through the high maximum torque.”
The development engine is based on the standard 134kW/240Nm, 1,596cc production unit with direct injection and a single turbo. To this Ford added a cylinder head with integrated exhaust manifold and the dual-stage boosting system, which only added 11kg. The control valves and bypass are water-cooled to ensure thermal robustness.
Kuhlbach believes that further improvements to performance and efficiency could come from a variable nozzle turbo used on the high-pressure stage and possibly even adopting the Miller cycle.
Despite the success being achieved with dual stage, axial turbine and multi nozzle turbochargers, there appears to be a resurgence of interest in the electric supercharger to improve response from downsized engines.
The role of electric supercharging.
With a response time of less than 350ms to spin the turbine shaft from idle to 70,000 rpm the electric supercharger (commonly known as a Variable Torque Enhancement System) could be very effective in addressing the low-speed turbo lag issues associated with downsized engines.
Having acquired the switched reluctance motor electric-supercharger technology from Controlled Power Technologies in December 2011 Valeo intends to launch production of its new electric supercharger in 2015-2016.
The electric supercharger can be used as a single stage system on small displacement engines, from 1.0L to 2.4L; or it could be used as the low pressure stage in a two-stage system. (Valeo have completed engineering work on turbocharged gasoline and diesel engines from 1.0L to 4.0L.)
When fitted to a Ford Focus HyBoost project vehicle equipped with a 1.0L, 110kw 3-cylinder EcoBoost engine, the combined fuel economy using the standard European driving cycle was improved by between 41% and 52%.
It must however be noted that the demonstrator car employed numerous electrification technologies, including: a stop-start system, floating voltage electric recovery system with ultracapacitors with a dc/dc converter, gasoline cooled EGR system, an air intake module with embedded water-cooled air charge cooler.
In addition to 12V architecture, the electric supercharger is compatible with up to 27V systems using ultracapacitors, and crucially for future developments, the electric supercharger will also be available for 48V applications.
Downsizing and turbo charging in Formula 1.
With the introduction of the new technical regulations governing Formula 1 from 2014 downsizing and turbo charging is a much discussed development. Faced with the challenge of maximum performance and reduced fuel consumption, while confined to a single turbo charger with a Motor Generator Unit-Heat (MGU-H), teams have taken the VTES a step further and applied this to the massive turbo system needed to boost in excess of 3,5 Bar.
With the MGU-H connected to the turbocharger it generates power from the turbine shaft to recover part of the heat energy from the exhaust gases. Part of this electrical energy is used to control the speed of the turbocharger to match the air requirement of the engine: both to slow it down in place of a wastegate, or to accelerate it to compensate for turbo-lag: It’s estimated that the system reduces lag fro 10’s of seconds to milliseconds.
Ensuring the durability of downsized turbo engines.
Downsizing and turbocharging with increased specific power output is not merely about smaller displacement engines with fewer cylinders being fitted with forced induction: Reliability and endurance cannot be compromised. This requires advanced/ new technology in several areas; including engine construction.
Cylinder bore distortion is one of the main factors limiting increases in power and torque outputs, particularly in lightweight aluminium cylinder blocks. Federal-Mogul’s patented Hybrid Liner increases the strength and stiffness of the combined block and liner assembly, allowing significant weight reduction without compromising engine performance and durability.
The Hybrid Liner reduces bore distortion in a running engine by two-thirds, while maximum second order bore distortion can be as low as 11 percent of that experienced with corresponding cast-in liners, and the cylindrical distortion under operating loads is up to three times better. As a result, oil consumption can be reduced by up to 40 percent.
Importantly in a turbocharged engine, the Hybrid Liner also achieves a 30 percent higher heat transfer rate, reducing the cylinder wall temperature by up to 40°C compared to alternative designs. Dynamic strength is also increased: In a cyclic pulsing pressure test, the Hybrid Liner showed no failure at pressures of up to 200 bar, whereas a standard-liner block design cracked at 100 bar.
With Hybrid Liners allowing a material wall thickness of just 3mm between the bores the distance between adjacent cylinders, in pressure die-cast engine blocks is reduced, thereby significantly improving packaging.
With forced induction, the quality of fuel is critical to optimised performance and durability.
To date most testing in Europe has been conducted using European-standard 95 RON petrol; however little has been done to evaluate the effects of various Research and Motor Octane Numbers on highly-downsized turbocharged engines.
This is a topic of significant interest to the industry at present; in attempting to understand this, Shell have, for several years used a mathematical treatment of the interaction of fuel RON and MON in relation to engine load to adopt an octane appetite weighting-factor approach.
Despite the mathematical models, a question mark still hangs over the compatibility of lower quality and alternative fuels available in other markets when used in downsized forced-induction engines.
The question many in the industry are asking is: Are there any technologies that might find exclusivity in the downsizing and forced-induction race?
It may be easier to identify a few trends that may ultimately lead to certain technologies coming to the fore:
• Inline four cylinder engines are likely to find favour in the near to midterm, thereafter further downsizing to “super” three cylinders is likely to prevail
• To retain transient response dual or multi stage turbos are likely to win the day. With the predicted 48V electrification an electric supercharger could power the low pressure stage
• Direct injection (If the particulate issues are resolved) and EGR are also likely to dominate
What is definite is that downsizing and forced induction are here to stay!