According to an official Nissan source, the successful development of variable compression engine technology represents a breakthrough in powertrain development that essentially, sets a new standard in terms of engine/powertrain capability, performance, efficiency, and fuel economy. In this article, we will briefly discuss how INFINITI’S turbo charged 2.0L variable compression engine works and what its claimed advantages are, starting with this question-
Put simply, a variable compression engine is one in which it is possible to alter the effective stroke while the engine is running under load in order to create a range of compression ratios, which in the case of INFINITI’S VC-Turbo engine, range seamlessly from as low as 8:1 to as high as 14:1.
In this particular engine, which incidentally, will not reach the production stage until 2019, the ability to change the effective stoke length by up to 6mm resolves many, if not most of the issues that come with forced induction. For instance, at the lower 8:1 compression ratio, there is no need to retard the ignition timing to prevent detonation (aka pre-ignition or knocking) at high boost pressures, while at the higher 14:1 compression ratio, smaller injection volumes coupled with the Atkinson cycle increase power while using less fuel, which in turn, produces fewer emissions.
Unlike other attempted designs, and most notably that by Peugeot, which altered the effective length of the connecting rods, INFINITI’S engine uses a device known as a “multi-link”. In practice, this device fits on the crankshaft where the connecting rods would be attached to the crankshaft on a conventional engine. What makes this arrangement work in the sense that the piston still imparts a rotary motion to the crankshaft is the fact that the connecting rod supplies one energy input, while the opposite end of the multi-link’s connection to the crankshaft is anchored. Seeing that the centres of crankshaft journals are at the exact midway point between the connecting rod’s attachment point and the anchoring point, the piston’s reciprocating motion imparts the same rotary motion to the crankshaft as the pistons on a conventional engine would have done.
However, the multi-link’s anchoring point is in its turn, connected to a rotatable shaft that in its turn is connected via a control arm to an actuator known in Nissan-speak as the Harmonic Drive. While this might appear to be very complicated, the trick to understanding how this engine works is to visualise the Harmonic Drive changing the angle (relative to the horizontal plane) of an imaginary line drawn through the centre of the crankshaft journal, and the centres of both the connecting rod’s attachment point and the anchoring point.
In practice, the Harmonic Drive rotates the shaft that anchors the multi-link, which changes the angle described above relative to the horizontal plane. Since the connecting rod is attached to the opposite end of the multi-link, the piston’s travel must necessarily change as well, and depending on the direction of the change, the compression ratio of all the cylinders will either increase or decrease by an equal amount, since all the pistons are connected to the multi-link.
It must be noted though that once the Harmonic Drive has altered the compression ratio, the mechanism is immune to vibrations that may cause unintended or uncommanded changes in the effective stroke length. The only time that any given effective stroke length will change from a current setting is when the ECU commands such a change, which depends on input data from various sensors such as the throttle position sensor(s), engine speed, actual (as opposed to desired) turbo boost pressure, engine coolant temperature, and vehicle speed, among other parameters.
While the actual technology has been shown to work well in a prototype engine, making it work as well as it does required some other refinements and adaptations to existing technologies, including the following-
Ability to switch between the Atkinson and regular 4-stroke cycles
“Smart” variable valve timing allows the engine to operate the Atkinson cycle under high-compression conditions. In this cycle, the air intake and fuel injection events overlap slightly, allowing for greater expansion of the air/fuel mixture, which improves combustion through increased volumetric efficiencies. The Atkinson cycle is in common use on hybrid vehicles because the cycle improves the efficiency of relatively small capacity engines.
However, as the compression ratio on the INFINITI’S engine decreases, the ECU seamlessly reverts to the normal 4-stroke cycle to increase engine efficiency under conditions where high boost pressures do not work well with the Atkinson cycle.
Twin fuel injection system
Under high compression conditions, the engine uses the direct fuel injection system to increase efficiency, while at the same time, avoiding premature ignition of the air/fuel mixture at high loads while the compression ratio is at the upper end of the scale.
At lower compression ratios, the engine uses the multi-point fuel injection system to allow the air/fuel mixture to mix earlier, which increases engine efficiency at low engine speeds and loads.
The practical advantage of having two separate fuel injection systems is that the engine’s efficiency and power delivery characteristics can always be maximised, regardless of operating conditions. Note that due to improvements in the engine management systems’ software, it is possible that both fuel delivery systems can be used at the same time under some conditions, with the split between multi-point and direct injection depending on the engine load and speed.
Integrated exhaust manifold
While exhaust manifolds that are integrated into the cylinder head is not exactly a new development, the combined effects of an integrated exhaust manifold and variable compression has been described as “being greater than the sum of the individual advantages of each of the components on its own”.
For instance, the single-scroll turbo charger has been matched to the flow characteristics of the exhaust stream on this particular engine, which has largely eliminated turbo lag, partly due to the use of an electronically controlled waste gate that can be more accurately controlled than a vacuum-operated unit can be. Moreover, since the catalytic converter and the turbo charger essentially form a single assembly that is bolted straight onto the cylinder head, throttle response is vastly improved, while engine and catalytic converter warm-up times have been greatly reduced.
High-tech cylinder wall coatings
According to official sources, friction between the pistons/ piston rings and the cylinder wall has been reduced by as much as 44% by the application of a proprietary coating on the cylinder walls. While this should allow the engine to rotate more easily, there is no information available on the design of the piston rings that have to contain the combustion pressure while sliding over as Nissan refers to them, “ultra smooth” cylinder walls.
Reduced engine noise/vibration
Again, according to official sources, the introduction of the multi-link system has allowed engineers to do away with the two balancing shafts used in other Nissan engines. The practical advantage of this is that the variable compression engine’s vibration and noise levels approach those of most V6 engines, which are inherently well balanced. However, it is more likely that the reduced vibration on this engine is the result of the fact that the connecting rods remain close to vertical as the engine rotates, which necessarily reduces the sideways thrust on the pistons.
This engine is also the first to be fitted with engine mounts that actively produce counter forces to damp out engine vibrations. Known as Active Torque Rods (ATR’s), these mounts contain sensors that detect vibrations, which are then damped out by a proprietary actuator that creates equal and opposite reactions, resulting in an overall reduction of engine noise by about 9 decibels.
As a practical matter, this engine offers a compelling alternative to diesel engine technology, in the sense that the combination of efficiency and power delivery represents a meaningful challenge to the idea that high torque values from relatively small capacity engines is the exclusive preserve of diesels and hybrids.
For instance, Nissan claims that this engine develops 200 kW at 5,600 RPM, and 380 Nm of torque between 1,600 RPM, and 4,800 RPM. These figures are higher than many, if not most turbo-charged 2.0L petrol engines from competing manufacturers, and compare favourably with several contemporary V6 engines. According to Nissan’s chief powertrain engineer, the production version of the 2019 INFINITI QX50 will outperform all of its four-cylinder competitors from 0 to100KM/per hour by about one second, largely because of the fact that turbo lag on this engine has been eliminated.
Moreover, Nissan claims that the variable compression engine in the 2019 INFINITI QX50 will deliver improved fuel economy as well. For example, during fuel consumption tests in the USA, the prototype delivered figures of 11.47 km/litre in front-wheel drive mode, which represents an improvement of 35% over the outgoing V6 engine in the current QX50. In AWD mode, the prototype delivered fuel consumption figures of 11.05 km/litre, which represents a 30% improvement over the same outgoing V6 engine.
However, although the 2019 INFINITI QX will only be offered with a CVT (Continuously Variable Transmission), Nissan has not released any data on the contribution the transmission has had on the claimed fuel consumption figures.
Although Nissan’s successful development of a variable compression engine represents an objective breakthrough in engine design, many commentators are of the opinion that it is unlikely to have a marked impact on global vehicle sales, despite its clear advantages over conventional, fixed-compression ratio engines.
Most major manufacturers have shifted their focus towards hybrids and purely electric vehicles, which means that the new technology may be nothing more than a stopgap measure to fill the gap between the demise of the internal combustion engine, and the advent of practicable electric vehicles.
