This article is a version of the original technical article published by Matthias Penzel, Vincenzo Bevilacqua and Thomas Raab in Porsche Engineering Magazin 1/2017.
The V8 engine has had a long standing relationship as the go to configuration for a lot of power, in a reasonably compact structure. Due to the layout of the two banks with an offset between the cylinder centres, the length of a V8 engine is only marginally longer than that of an inline four cylinder, allowing its accommodation in the front bay of sports cars and SUVs, without causing either the long front hood or overhangs associated with V12’s.
Inherent advantages of the V8 are the distribution of a sizeable volume (e.g 6.3-litres) over many cylinders, evening out power and torque delivery. So the classic V8 represents a good compromise, offering small structural space requirements with a simple engine architecture, high power-to-weight ratio and extremely smooth running characteristics.
Typical V-engines are characterised by opposing piston-rods sharing a connecting pin. Theoretically, the angle between the centrelines of each bank of cylinders gives the bank angle (e.g 90-degree). Theoretically it can be lower than that (60° on the Yamaha-engineered Volvo XC90 V8’s of the last generation) or pushed outwards all the way to 180° or flat. The difference between a V8-engine laid out like that and a typical boxer engine is that the latter has independent connecting pins for each piston rod, alternating at 180°.
For this reason, the Horizontally opposed (Boxer) engine in modern architecture has more main crankshaft bearings than a comparable V engine. The usual number of main bearings today is:
– for V engines = (number of cylinders/ 2) + 1
– for Boxer engines = number of cylinders + 1
This in turn results in a further difference in the offset of the two cylinder banks: in a V engine, the bank offset is determined by the width of the connecting rod, while in a flat-plane engine it amounts to half the distance between cylinders.
The Bank angle dictates the compactness of an engine’s architecture and centre of gravity. For a four-stroke V8 engine, that means: 720-degree cycle angle, i.e. two crankshaft revolutions for a complete working cycle, divided by the number of cylinders (8) yield a 90° bank angle or a whole-number multiple thereof.
Design of the crankshaft
In the basic design of a V8 engine, designers have another important bit of room for manoeuvre: the configuration of the crank throws on the crankshaft. This has a crucial influence on the principal characteristics of the engine—whether sporty/aggressive or with comfort-focused smoothness and low vibrations.
The decision regarding the arrangement of the crank throws is shaped by the dichotomy between maximum power potential and optimal balancing of the free inertia forces and torques. Due to the kinematic coupling in the crankshaft drive, the inertial forces are produced by the oscillating motion of the piston and connecting rod masses. Depending on whether these inertial forces are produced one or two times per crankshaft revolution—for example through the upward or downward motion of the piston—we speak of primary and secondary forces in relation to the engine speed. If for the free inertial forces there is also a moment arm with respect to the engine center, this produces free inertia torques.
As the engine speed rises, free inertial forces and/or torques are felt in the form of increased vibration, which, particularly as primary and secondary forces, are perceived as unpleasant and can only be partially mitigated through the engine mounts. For the most part, conventional V8 engines feature one of two crank variants: the “flat-plane” crankshaft in which all crank pins are on a single plane, and the “cross-plane” crankshaft, in which the crank pins of the four cylinder pairs are arranged at 90° angles to each other.
Cross-plane V8 : Emotional sound
The cross-plane V8 has a characteristic burbling sound. However, this sound is the result of an incomplete ignition process, caused due to the proximity of adjacent cylinders at different phases of their firing cycle. To overcome this, complex manifold geometries are needed to route residual gas.
The residual gas can also cause explosive knocking, that can damage pistons, needing a knock control system.
A V8 engine with a cross-plane crankshaft experiences this problem in a particularly pronounced form. In spite of the generally even ignition order in the engine as a whole, with a 90° bank angle there is still an uneven firing order in each cylinder bank. Two cylinders per bank always fire in direct succession (90° ignition interval). What that means in real terms is that the exhaust pressure pulse of the subsequent cylinder occurs while the exhaust valves of the previously ignited cylinder are still open. As a result, exhaust is pushed back into these cylinders, which in turn adversely affects the quality of the gas cycle.
As part of a current V8 engine project, Porsche Engineering has now broken new ground in this context. With specific control times for each individual cylinder, the residual gas problem can be eliminated with minimal effort.
The cross-plane V8 engine typically earns high marks in two other important categories: smoothness and low vibrations. In terms of free inertial forces and torques, the cross-plane configuration is ideal. While there is a remaining primary free inertial torque, this can be relatively easily counteracted through balancing masses on the outer counterweights of the crankshaft. The result is perfect balance.
The double four-cylinder: flat-plane V8
The flat plane crankshaft is like one on a typical 4-cylinder engine, except for the two connecting rods on each pin. The similarity to a four-cylinder is no coincidence. The at-plane V8 embodies the original idea that led to the development of V8 engines, i.e. combining two inline four-cylinder engines in an angled configuration. And this is what gives rise to the fundamental advantages and drawbacks of this configuration. The secondary free inertial forces of the four-cylinder are retained and combine along vectors in the V configuration. The gas cycle, on the other hand, is considerably more harmonious. The ignition in a flat-plane V8 jumps from one cylinder bank to the other, which eliminates the residual gas problem of the cross-plane V8. The even, alternating expulsion of the exhaust also produces a completely unique engine sound that sounds noticeably like that of two inline four-cylinder engines—penetrating and aggressive.
Differing ignition sequence depending on the manufacturer
While the firing order determines the crankshaft rotation angle traveled between the ignition of two cylinders, the firing order defines the unique sequence of the cylinders in succession. As fixed geometric variables, the bank and crank angles only allow certain orders. The respective configuration defines which pistons reach their top dead center. The firing orders of flat- and cross-plane engines therefore differ in principle. Nearly all modern flat-plane V8 engines fire in identical sequences; in cross-plane V8 engines, by contrast, one generally finds manufacturer-specific firing orders. This takes into account a circumstance that can lead to slight confusion: worldwide there are different definitions as to which cylinder is counted first and how the other combustion chambers are numbered. This would seem to result in different firing orders. Removing the effects from the different cylinder counting methods, the variance in firing orders drops markedly.
If one begins the cylinder count in each case with cylinder 1 according to DIN 73021, there are a total of eight theoretically possible firing orders for each rotational direction in a flat-plane V8. With a cross-plane engine, the total is 16, as here the angle position of the center crank pin is interchangeable. However, not every theoretically possible ring order is implemented in reality. The objective is always the best-possible compromise between the following criteria:
- Gas cycle
- Stress on the main crankshaft bearings
- Vibration stimulation of the crankshaft drive through deformation of the crankshaft under loads
- Rotational irregularities