Don’t believe the hype about the death of diesel, says Richard Simpson; there’s at least another two generations of diesel drivelines under development, and they will be cleaner and more efficient than ever
Diesel is doomed. Or at least, that’s what you’d think if your knowledge of the subject was gleaned exclusively from newspaper headlines. And it’s true that, in the world of urban passenger transport, we are going to see an increasing number of vehicles powered by gas or battery. But the same does not apply to coaches. There is no capacity on a coach for the 12-tonne battery that would be needed for it to do an electrically-powered day’s work on the open road. And designing a coach that could accommodate enough methane without taking up all the luggage space would also be a bit of a challenge, to say nothing of the safety implications of perching 57 passengers on top of a magazine of gas tanks. [wlm_nonmember][…]
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So, diesel it is then. And for all the ‘dirty diesel’ trope, and high-profile statements about the end of Dr Rudolph’s infernal invention, the truth is that there are at least two more generations of heavy-duty compression ignition engines under development in Europe now.
But, the challenges are there too. European legislation demands that carbon emissions from heavy vehicles must fall by 20% by 2025, and 35% by 2035.
So, as they used to say on Alas Smith and Jones: “How’s that going to work, then?”
Efficiency is everything
And the answer is, more than likely, by removing waste. Wasted energy from over-fuelling, internal friction and unnecessary idling. Wasted energy from exhaust heat and sound, and friction braking. Energy wasted by driving auxiliary systems such as power-steering pumps. And energy wasted by dumping surplus electrical output and compressing air that is not needed.
The heart of any diesel engine is the fuel-injection process. And here developments to save fuel and improve combustion efficiency continue apace. Delphi, the American automotive company which inherited the old Lucas fuel-injection business in Park Royal, London and Stonehouse, Gloucestershire is putting its new DFI 21 injector into limited production for trials with engine manufacturers thought to include DAF and Volvo. DFI 21 is the key component in Delphi’s upcoming F3 common-rail injection system that will replace the F2 system which took DAF and Volvo to Euro VI.
In an extraordinary feat of miniaturisation, DFI 21 features an injector pin that is just 1mm in diameter and machined to an accuracy of a few microns: it allows the injector to deliver fuel in up to 10 separate doses per combustion cycle at pressures of up to 3,000 bar.
This lowers peak combustion pressures, reducing the formation of NOx and soot and reducing the burden on exhaust aftertreatment systems. Significantly, this will improve engine efficiency as less fuel will be wasted recirculating exhaust gases and regenerating particulate filters.
A further benefit comes from the miniaturised electronics, which allow true closed-loop control of individual injectors. Any mechanically-induced variation in injector performance will be corrected by the electronics.
This electronics expertise will also be used in hybrid drivelines. Delphi has no interest in developing its own hybrid: reasoning that its expertise lies elsewhere; but it will be ideally positioned to provide key components such as inverters to manufacturers of hybrids. It cites its Viper liquid-cooled high-voltage switch as an example of its expertise in the field.
And the use of high-voltage components to supplement conventional diesel drivelines can only grow. Delphi points out that a 48v mild hybrid can reduce the CO2 output of a conventional diesel by up to 15%.
One of the big energy savings in hybridisation comes from the recovery of energy that would otherwise be lost to friction braking by using the electric motor as a retarder/generator.
Electric power is particularly valuable in conditions where vehicle speeds are subject to cyclical variations. Diesel engines are at their most efficient in steady-state conditions, but energy is wasted on the over-run, and cannot be recuperated when the vehicle could use it to accelerate back up to cruising speed.
This is an area which is attracting considerable interest from leading heavy-vehicle component suppliers. Federal-Mogul Powertrain estimates that even a small motor/generator of 12kW integrated into a conventional coach or bus diesel driveline could save up to 8% fuel.
War against waste
There are more savings to come from finding waste elsewhere. Engine manufacturers themselves are focussing on energy lost in pumping and churning the lubrication oil and will specify ultra-low viscosity lubricants that meet the new API FA-4 (USA) and F11 (European) standards. The downside of these lubricants is that lower viscosity means a thinner oil film, and this requires the re-engineering of key components such as piston-rings and plain bearings. Federal-Mogul had developed a new coating material, dubbed IROX 2, for the aluminium and bronze plain bearings typically found in medium and heavy-duty diesel crankshafts. Not only does IROX 2 facilitate the use of low VI oil, it also, in its own right, reduces crankshaft friction thanks to an ultra-smooth finish. It has polymer resin binder that has key additives dispersed through the film which act as solid lubricants. The product is so effective that Federal-Mogul claims that it actually reduces friction more than the reduction from 5W-30 to 0W-20 oil that it facilitates.
Controlling such a thin oil on the cylinder wall is another challenge, and here Federal-Mogul has come up with what it calls the eLine design for the second piston ring, which retains a uniform film of oil around the circumference of the bore, even when a low-viscosity lubricant is used thanks to an e-shaped groove at its base.
The ring remains more stable than a conventional tapered design, and reduces blow-by by up to 20%. This translates directly to an increase in engine efficiency and a reduction in fuel consumption.
There is further scope for reducing parasitic drag from the engine by moving from mechanical to electrical drive for components such as compressors, power-steering and oil pumps. Not only does this allow their output to be controlled to suit demand rather than being dependent on engine speed, it also means that no fuel is wasted by inputting more energy than is required.
There’s yet more savings to come from the exhaust stream, according to Federal-Mogul. Conventional turbochargers can reuse up to 30% of the energy that is otherwise blown out of the exhaust, but there’s scope to be had for mounting further turbines in the exhaust downstream to capture some of the 70% of waste energy.
The difficult part is preventing this from upsetting the engine’s performance: Federal-Mogul’s TIGERS (turbo-generator integrated gas energy recovery system) uses an exhaust-driven turbine to drive a liquid-cooled switched reluctance generator to convert exhaust gas energy into electricity.
Depending upon the load on the drive train, this can either be used to power a COBRA (controlled boost for rapid response application) liquid-cooled electric supercharger to increase air flow into the engine (and thus power), or return energy direct to the driveline by means of the company’s SpeedTorq electric assist motor/generator.
Energy for this system is more likely to be stored in capacitors (which can accept and discharge large amounts of current in a short time) than batteries. However, the system is modular and scalable, making it suitable for a wide range of applications, and can operate at 12, 24 or 48volts.
Hybrid options
Valeo now offers two hybrid systems that have the potential to reduce fuel consumption by up to 8%.
The first is a 48-volt starter-alternator, which, being belt-driven, can be fitted to existing designs with relative ease. Dubbed the iBSG, it operates as a retarder during braking while charging a 48-volt 300Wh battery, and can provide up to 8kW of continuous power which can either be used to boost the engine’s tractive effort or drive electrical systems on the vehicle. It replaces both the conventional alternator and the starter motor, and in trials with a maximum-weight two axle truck produced fuel savings of between 3 and 5%. A lighter vehicle would see the system save a greater percentage of fuel.
Valeo’s second development is a 48-volt gearbox-mounted motor generator, which has a peak output of 15kW. In a light vehicle such as a minibus it has the potential to reduce fuel consumption by up to 20%.
Respected automatic transmission manufacturer Voith will offer a hybrid option on its DIWA Nxt, due to commence production in 2021. This can have a 48-volt energy recovery unit fitted into the flywheel between the engine and transmission. It offers 25kW continuous power and a maximum output of 35kW and can also power vehicle systems via a 48/24-volt DC/DC converter.
On the manual transmission front, there is a hybrid module available for the ZF Traxon automated unit. Traxon can be specified on many more powerful coaches. ZF claims that trials with it installed in a heavy truck produced fuel savings of between 5 and 7%, and the electric motor is powerful enough to move the vehicle at low speeds without running the diesel engine at all.
Eaton, which despite its relatively low profile in the European original equipment market was a prominent exhibitor at last year’s IAA Show, says it can install 48-volt motor generator into its Endurant heavy-duty automated manual transmission via the power-take-off port. This can replace the engine’s conventional alternator and starter-motor as well as driving the power-steering pump and potentially gathering energy for other systems including ‘engine off’ cabin climate control. Not only will this save fuel, it also shaves about 22kg off the installed weight of the engine-transmission assembly and removes the clutter of drive-belts from the front of the engine.
In the North American market, its Endurant AMT is specified with different engines including Volvo and Paccar (DAF), but it has a significant partnership with Cummins, with the transmission supplied mated to the X12 12-litre straight-six diesel.
Cummins itself obviously sees a healthy future for hybrids. In the last couple of years it has acquired the battery system division of respected UK automotive supplier Johnson Matthey, North American electric motor manufacturer Brammo and Silicon Valley hybrid specialist Efficient Drivelines Incorporated.
The fruits of these takeovers were to be found in the prototype Cummins PowerDrive truck shown at last year’s IAA. Its versatile powertrain has clear applications in touring coaches as it allows 50 miles of electric-only operation for environmentally-sensitive areas (such as historic tourist centres where diesel coaches may not be welcome for much longer) and hybrid diesel-electric running elsewhere.
It combines both series and parallel hybrid drivelines. In traffic and at low speeds, where power demand is relatively low, the hybrid operates in series mode, which is to say that the vehicle is directly powered by its electric motor only, and the Cummins B6.7 engine runs only when it is needed to top the batteries up. In situations where power demands are high, the vehicle is directly driven by the combined power of both the electric motor and the diesel engine. Zero-emissions battery charging can be provided when the vehicle is parked, and it can also charge other electric vehicles if required.
Complete package
At last year’s Euro Bus Expo in Birmingham, Scania showed a complete parallel hybrid chassis for a suburban commuter coach. The K320EB combines a 9-litre 316hp engine and an electric motor in the transmission. There is a stop-start system for the diesel engine to save fuel in heavy traffic. Cummins also offers stop-start on its B4.5 and B6.7 engines.
Engine stop-start on larger non-hybrid diesels could be a step closer thanks to an innovation from Jacobs – a company best-known for its compression-release engine-braking systems. It uses its valve manipulation technology to allow an engine to be spun up to starting speed in a decompressed state, reducing both the load on the starting system and the vibration felt by the vehicle’s occupants.
Jacobs can also use the technology to completely deactivate selected cylinders on a large lightly-loaded engine at normal running speeds. One, two or three cylinders can be deactivated as required when only small amount of engine power are needed. This reduces the effort required to pump air through the engine in low-load conditions, but also give a bonus in increased temperatures in the exhaust stream. This enables optimum function of exhaust aftertreatment emission-control systems with temperatures maintained without recourse to fuel-wasting post-combustion diesel injection, or forced regeneration of particulate filters.
Surplus heat in the exhaust stream can also be recovered and turned into useful energy. Mahle, an engine component manufacturer best known for pistons, is committed to continue its diesel development programme until at least 2030. It now offers insulated pistons to retain as much heat as possible in the exhaust stream and a so-called Boost Box recovers waste heat from the exhaust and turns it into a 48-volt electric supply.
Weighing 150kg and occupying a 135-litre volume, the Boost Box can return 13kW to the powertrain or use the electric power for auxiliary systems. Mahle claims the Boost Box itself it retrofittable, although plumbing-in the 48-volt electrics into an existing vehicle would be more of a challenge.
All the technologies described above have potential to either give today’s engines a fresh lease of life, or enable the development of smaller, lighter and more efficient new designs. But more significantly manufacturers would not be ploughing significant R&D resources into these devices if they thought that time had run out on the diesel engine and it had become the technological dead-end that it is sometimes portrayed as.
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