Chances are that you may have heard or read that some major car manufacturers are dumping their V6 engines in favour of inline six-cylinder configurations, which begs this question- why? After all, V6 engines have served their makers well over the years, so why dump a tried-and-tested engine design? The answer is rather complicated and while some commentators are reducing the question to a simple “V6 equals bad, and inline six-cylinder configurations equals good”, the real answer lies somewhere between the two extremes. Thus, in this article, we will take a closer look at the pros and cons of each engine design in terms of their inherent characteristics, starting with this question-
The short answer is that there are no really bad V6 engines anymore, but more to the point, when the question of which is better- V6, or inline engines arises, it should be borne in mind that both engine designs have advantages and disadvantages. Therefore, the issue must be decided by comparing apples with apples, so to speak, and not based on personal preferences. Let us look at some of the issues that make V6 engines great -
V6 engines are good for weight distribution
Since V6 engines are significantly shorter and more compact than inline six-cylinder engines with similar displacements, it is possible to locate a V6 engine further behind the front axle line than is possible to do with an inline engine, thus greatly improving the overall distribution of weight between the two axles.
V6 engines can be used in front wheel drive applications
There are many examples of V6 engines being mounted transversely in FWD models, but in almost all cases, the decision by car manufacturers to produce these examples had more to do with cost savings and gaining market share over competing manufacturers than with any inherent advantage(s) V6 engines might have over inline engines.
For instance, it is much cheaper to beef up the suspension of a 4-cylinder model to carry the weight of a V6, than it is to develop a new model from the ground up. In fact, since many manufactures use a single platform to expand model ranges, it is a simple matter to add slightly upgraded brakes, a strengthened suspension system, and reprogrammed electronics to a V6 engine to produce an instant V6 variant of a model that started life with a 4-cylinder engine.
So, considering the above advantages of V6 engines, why are some car manufacturers dumping them? There are several very good reasons for this, but let us start with this-
V6 engines are expensive to produce
While no vehicle manufacturer ever publishes analyses on what it costs to produce engines, the fact is that a V6 engine requires three castings; one engine block, and two cylinder heads- and sometimes a separate sump casting as well.
Two cylinder heads translate into twice the engineering time it takes to finish a single cylinder head, to which must be added the cost of additional camshafts and other timing components, as well as additional labour costs associated with assembling a V6 engine.
At first glance, the additional costs might not appear to be problematic, but if a manufacturer produces say, 50 000 or more V6 engines per year, the costs add up to millions- millions of dollars or Euros (or whatever currency applies) that many manufacturers are now pouring into the development of hybrid and/or electric vehicle technologies.
The above should give you some idea of why V6 engine are falling out of favour, but what is it that now makes inline six-cylinder engine a more attractive proposition? There are two principal factors at play- one that involves improved technologies, and one that involves the question of balance. Let us look at-
There was a time when ancillary equipment like water pumps, radiator fans, and A/C equipment were hung onto the front-ends of inline engines, which contributed significantly to the overall length of these engines. By contrast, the compactness of V6 engine usually left plenty of space in front of the engine to place these components
Moreover, since V6 engine are generally very wide, it has become increasing difficult to locate performance-enhancing equipment like turbochargers, and especially sequential turbochargers, in engine compartments that have become smaller and more cramped. As a rule and since exhaust manifolds took up most of the space on either side of V6 engines, the only available space in which to locate many performance-boosting components is in the valley between the cylinder heads, in space that is required to fit an intake manifold.
Now however, electric water pumps, electrically operated A/C compressors, and electrically operated turbo chargers can be placed almost anywhere alongside an inline six-cylinder engine. A good case in point is the electric turbocharger that was developed to work with Mercedes-Benz’s new inline six-cylinder engine. This turbocharger, which MB claims eliminates turbo lag completely, can spool up to 70 000 RPM in less than 300 milliseconds, will be used in a sequential set-up with a normal variable geometry exhaust-driven turbocharger.
Such a set-up would be extremely challenging, if not impossible to achieve on any V6 engine in use today because to make such a set-up work to its full potential, you’d need two exhaust-driven, and two electrically operated turbochargers. How the intake and boost pressure ducting for such a set-up would be routed around a V6 engine in a cramped engine compartment is however, anyone’s guess.
Up to this point, we have only discussed some peripheral issues, most of which the average driver or car owner neither knows about, nor cares about. However, studies performed by several consumer organisations indicate that the most important consideration for most buyers of new vehicles is (somewhat surprisingly) not reliability, but refinement. From a consumer perspective, the concept of refinement has many aspects: for some people it is the seating position, for others it is the layout of the dashboard and controls, and for still others, refinement has more to do with how smoothly the engine runs than with anything else.
Car manufacturers on the other hand, and especially manufactures of midlevel, to high-end vehicles, know that a), different people look for different things in a new vehicle, and b), that the easiest way to accommodate most consumer preferences is to produce a vehicle that is essentially free of vibrations, which brings us to-
This image encapsulates the biggest single drawback of V6 engines that are not angled at 120 degrees, which is the fact that pistons move in different directions, thus causing significant plane imbalances in both their rotating and reciprocal masses, as well as rotational imbalances during compression and torque generation. For the most part, these imbalances are more pronounced in V6 engines like the one above, in which two pistons share a crank pin or bearing journal. Moreover, despite the obvious advantages of a short crankshaft and increased structural integrity that comes with a compact engine block, the fact that the two cylinder banks on V6 engines are staggered makes it particularly difficult to counter reciprocal plane imbalances with massive crankshaft-mounted counterweights.
Nonetheless, two examples of well-balanced V6 engines are Alfa-Romeo’s 2.5L and 3.0L engines, in which lightweight pistons in combination with highly engineered engine mountings and counterweights mounted between the “split” crank pins all but eliminated both rotating and reciprocal imbalances. These engines are the exception however, and most V6 engines in use today suffer from inherent first and second order vibrations that are very expensive, not to mention challenging from an engineering perspective to eliminate, which begs this question-
Well, yes and no. Yes, because inline engines have virtually no reciprocating mass and rotational plane imbalances, and no, because inline engines suffer from high secondary imbalances* at high engine speeds.
* When a crankshaft rotates 90 degrees from the TDC position, say, #1 connecting rod is at its most tilted position when the rod’s big-end reaches the exact midway point in its rotation, which means that in practice, the small-end is somewhat below the midway point in the stroke at both 90 degrees and 270 degrees ATDC. The practical effect of this is that between 90 degrees BTDC and 270 degrees ATDC, the piston moves a shorter distance than it does between 90 degrees BTDC and 90 degrees ATDC.
Put in another way, this means that if the crankshaft rotates at a constant speed, the piston travels a shorter distance, and therefore, its rate of movement is higher in the top half of the crankshaft’s rotation than it is in the bottom half of the crankshaft’s rotation. As a result of this, the inertial forces created by the piston when it accelerates and decelerates are markedly stronger in the top half of the crankshaft’s rotation than in the bottom half.
The above is a necessarily brief explanation of the primary cause of secondary imbalances and vibrations in inline engines. In practice though, secondary imbalances are directly proportional to both the mass of the pistons and the overall engine speed, which means that at idle and low engine speeds, most modern inline six-cylinder engines are essentially vibration-free since secondary imbalances are almost entirely absorbed by three crankshaft throws that are spaced 120 degrees apart. Here is how it works-
From a design perspective, inline six-cylinder engines are essentially two three-cylinder engines that are joined together by a common crankshaft that allows the pistons to move together in three pairs. For instance, pistons 1 & 6 will always be move together, pistons 2 & 5 will always move together, and pistons 3 & 4 will always move together. Regardless of which stroke any given piston is on, the paired pistons results in an almost perfect reciprocal and rotational balance, which is further improved by an overlap in torque generation within every 120 degrees of crankshaft rotation.
Note though that while inline six-cylinder engines typically have some degree of plane imbalances during compression strokes, these vibrations can be minimised, if not eliminated by designing intake and exhaust manifolds so that both the intake and exhaust pulses are evenly distributed. This does not only reduce the intensity of reflecting/bouncing pressure waves in the intake manifold plenum and runners, but improves exhaust scavenging as well.
However, nothing is ever free, and the almost perfect reciprocal and rotational balances of inline six-cylinder engines come at the price of high secondary imbalances (discussed previously) causing significant torsional vibrations in both the crankshaft and camshaft(s). While these vibrations can be controlled by harmonic balancers, the best way to reduce torsional vibrations and stresses is to reduce the torque inputs on the crankshaft, which is usually accomplished by decreasing cylinder bore diameters and increasing effective stroke lengths.
Again, nothing is free and while smaller bores reduce torque inputs on the crankshaft, longer strokes increase secondary imbalances. As a practical matter, this means that the smoothness of modern inline six-cylinder engines is the result of complex compromises between bore diameter, stroke length, cylinder spacing in the block, crankshaft length, and the number and spacing of support (main) bearings. In addition, both the static and rotational masses of the flywheel (or torque converter assembly) and the harmonic balancer must be matched to both the natural vibration frequency of the crankshaft, and the range of frequencies induced (in the crankshaft) by torque inputs to prevent crankshaft failure.
Having said all of the above, the question now becomes-
In the final analysis, there is no single factor or characteristic that makes inline six-cylinder engines “better” than any modern V6 engine that is in use today. However, from engineering, design, and production cost perspectives, there is a host of reasons why inline six-cylinder engines are making a huge comeback. Let us look at some of these reasons-
Inline engines are becoming more compact
The advent of electric turbo chargers and water pumps now make it possible to reduce the overall length of inline engines, which in turn, makes it possible to place these engines further behind the front-axle line of cars than was possible to do before. This is particularly important in high-end applications, where the improved weight distribution makes for improved handling characteristics.
Inline engines are becoming cheaper to produce
Most new inline engines are made on the “modular” principle, which means that cylinder spacing is the same not only on all displacements, but also on V8 versions. In practice, this means that all engine block castings can now be machined and finished on the same tooling line, which translates into massive cost savings for manufacturers.
Inline six-cylinder engines are inherently better balanced than V6’s
While this is true, this statement must be qualified by saying that superior balance comes at the cost of severe displacement limitations, since bigger bores and heavier pistons translate directly into increased secondary imbalances. Nonetheless, displacement restrictions have been largely overcome by electronically controlled forced induction in combination with improved fuel delivery and ignition control strategies, as well as improved manifold designs that greatly improve both intake airflow and exhaust scavenging, which leaves us with this-
It is interesting to note that to date, only manufacturers of high-end vehicles, including Mercedes-Benz, Land Rover, and Jaguar have announced plans to dump their V6 engines in favour of inline designs. How much of this decision has to do with inline engines’ superior balance and cost of production issues, and how much of it has to do with consumer demand for vibration-free vehicles is not entirely clear, but it is worth noting that BMW has never abandoned six-cylinder inline engine configurations.
As technicians, we know that it is very hard to beat an inline six-cylinder BMW engine in terms of smoothness at idle and low engine speeds. So perhaps the dumping of V6 engines by major manufacturers is likely less of an effort to produce vibration free vehicles than it is about returning to the basics of managing vibrations in vehicles, while producing inline six-cylinder engines that can compete with many, if not most current V8 engines in terms of power output, at the same time.
