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From engine to wheels


48 volt systems, disappearing belts and evolving compressors: The internal combustion engine gets ready for new challenges

Nicodemo Angì

Although pinning all the blame of pollution on cars and other means of transportation isn’t fair, it is an undeniable fact that they play a relevant part in the emission of hazardous particles into the atmosphere. Countermeasures must be taken and that is precisely what the industry is doing, producing cleaner and more efficient new engines. This technological new wave is rather across-the-board, in the sense that it involves luxury vehicles as well as compact city cars sharing sophisticated engineering aimed at enhancing performance.


A class “straight-six” act                      


We start talking about the new Mercedes petrol engine, identified as M256, a 400hp 3-liter straight 6 engine, with a torque of 500 Nm, which will make its debut on the S-Class. Straight-six engines have a clean and elegant design because it is inherently balanced, even against forces of the second order, and therefore do not need a balancing countershaft. Another advantage compared to V6s is their greater efficiency and manufacturing simplicity: it is true that the shaft is longer but the engine block is more linear and the cylinder head is only one, although rather cumbersome. Likewise, the distribution is also simpler - 2 camshafts against 4, only one chain / timing belt - and this will positively reduce internal friction, resulting in increased efficiency. What is to be considered a rather classic design, is now been revisited by Mercedes, thanks to the adoption of a 48 volt electric system to power the main on-board services, including the turbo-charger. The M256, in fact, can boast a new generation "smart" turbocharger assisted by an electric compressor, which will almost eliminate any turbo-lag: the electric auxiliary compressor can, in fact, spool up to 70,000 RPM in just 300 milliseconds. The 48 volts electric system decreases by a factor of 4 the electric current given the same power, drastically reducing energy dispersion in the conductors and allowing to electrically power the water pump and A/C compressor with greater efficiency.  The 48 V power supply is also entrusted to an unusual component, an Integrated Starter-Alternator, an electric unit driven by the engine. The ISG acts as a starter motor (the Start & Stop is guaranteed to be extremely subtle), regenerates energy during braking and "aids" the internal combustion engine with a boost of 15 kW and 220 Nm, thus optimizing the operating point, thanks also to the Camtronic variable valve timing. These solutions allowed Mercedes to build what they call a “beltless” engine, shortening it to almost the same length of a V6 engine of the same displacement.


A “global” power unit

In a different category but equally developed and optimized is the new Firefly, identified as Global Small Engine, and recently developed by FCA with a team of Italian, Brazilian and American engineers. The GSE aims to rationalize the line of “small” engines of the Italian-American manufacturer. Designed in two versions, a 999 cc 3 cylinders and a 1,3 liter 4 cylinders, the GSE will be produced in Brazils as well as Bielsko-Biala in Poland from 2018 with an investment in the region of 250 million Euro. The Firefly, in a way, goes against the tide: with only 2 valves per cylinder and in the absence of a turbocharger, the one liter version still proves to be more powerful than the older 1,3 Fire that will be replaced: 77 hp (with ethanol, otherwise the power settles around 72 hp) against the 69 hp of the Fire. This improvement is the result of a high 13.6:1 compression ratio, which indicates how the engine works, at least at low loads, using the Miller cycle, (more commonly called a modified Atkinson). When the driver lets up on the throttle, with most engines, there are high pumping losses, with an obvious loss of efficiency. The Firefly, instead, changes the camshaft axis to delay valve opening, which reduces pumping losses. Other strategies to reduce the frictions are a single camshaft, provision for the use of low viscosity oils and valve gear run by a chain, for lower maintenance and more precise actuation over time. The centerline of the piston and crankshaft was offset, to cut friction between the piston and cylinder; and a variable-displacement oil pump was adopted. The search for a favorable torque at low revs has led to the adoption of a relatively long stroke and long intake manifolds.  The modular design of the Firefly means that, to increase the engine’s displacement, all is needed is to “simply” add a cylinder: block machining and cylinder boring as well as machining heads, crankshaft and components remain the same. The 1.3-liter version will also have an alternator-starter that activates the start & stop system and smart charging, which will take place primarily during the release phase to reduce consumption. We also noted that this line of engines is expected to be enriched by a turbo version with 4 valves per cylinder, which should be able to develop 120 and 160 hp, with special configurations for Alfa Romeo.


Intermediate balance

Between these two engines, rather distant in their intended use and design concept, lies the new 2.5-liter, naturally aspirated, Toyota engine. After being unveiled for the first time in the North American International Auto Show 2017 in Detroit on a Toyota Camry, this engine will likely equip also other cars, including Lexus. It is a 4-cylinder naturally aspirated “high-speed” combustion engine with variable valve timing equipped with a host of new technologies to favor great torque throughout the rev range and record breaking thermal efficiency: 40% in "normal" configuration and 41% for  the hybrid version. Record setting values indeed, even in the presence of more than adequate power: the classic version in fact produces 60 kW/liter, equal to 81.5 HP/liter, a value that drops to 52.2 kW/liter for the super efficient hybrid version. The expedients used to increase performances are many, starting from a compression ratio of 13 (14 for the hybrid version) and the long stroke which produces a good maximum torque at 4,800 rpm, well distant from that of maximum power, set at 6,600 rpm. The combustion has been particularly studied and optimized thanks to the straight intake ducts free of any obstacles, a large angle between the valves, new injector and a strong turbulence, which improves the fuel/air mixture. The EGR system is cooled, to prevent self-ignition, and there are the systems of variation of valve timing such as the VVT-iE and IN-VVT, capable of managing the engine in an Atkinson cycle. Services too, have been optimized to enhance efficiency, adopting a water pump, an electric thermostat and a variable displacement oil pump with a Trochoidal inner rotor.

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