The way in which an engine is orientated can have a huge influence on the rest of a car’s design. Should the car be rear-wheel drive? What size of engine is expected? Are there space issues that could influence which way the engine lies? All these questions have to be answered by the engineers responsible for the car, as the powertrain setup can have a large impact on how the car goes, handles and stops.
Looking at front-engined cars, the two possible orientations are longitudinal and transverse, with each having its own benefits and drawbacks that can govern which is chosen for the car at hand. In-light of this engineering decision, here’s a basic guide to the features of each engine layout and which is best for different applications.
Transverse engines are mounted perpendicularly to the direction of travel, lying horizontally within the engine bay. This is predominantly used in front-engined, front-wheel drive setups but has also been implemented in other engine placement options. A common transverse engine setup has the transmission bolted in-line with the engine (known as a transaxle), followed by a differential that has driveshafts protruding from either side.
These engines were thrust into the mainstream by the BMC Mini which utilised a transverse engine to maximise interior space. This design was particularly revolutionary as the transmission was engineered into the engine’s sump, making for a powertrain that took up a tiny footprint of space within the engine bay. This meant that the relatively powerful and ‘large’ displacement A-Series engine could be crammed into the tiny Mini’s chassis, producing impressive performance for its day.
As the need for bigger engines and more gears increased, transmissions moved to the side of engine blocks along with an off-centre differential for a simple insertion of the driveshafts through CV joints. Due to the transmission situating itself at one side of the engine bay, the driveshafts have to be different lengths to reach the wheels in comparison to a longitudinal setup that sits along the centreline of the car meaning the shafts are of equal length.
Transverse engines have become the norm in smaller mass-production cars, with a V6 normally being the largest engine layout that can be mounted in this way (although a transverse V8-powered cars do exist). The main reason they are so popular is the amount of space they free up elsewhere in the chassis. In city cars, a transverse engine allows for much more room in the interior to comfortably take five adults and some luggage. The lack of a central tunnel needed to leave space for the transmission and a propshaft to a rear differential (a longitudinal RWD system) means that the floor can be much flatter to maximise passenger comfort.
In terms of driving dynamics, a front-mounted transverse engine places the majority of the car’s weight over the front wheels. In a FWD setup, this makes for maximum traction for the driven wheels which is obviously advantageous for acceleration and for tackling slippery surfaces. The lack of drivetrain components needed also means that overall weight can be decreased and the manufacturing costs reduced, making these cars cheaper in general.
Unfortunately, transverse engine layouts do have their limitations. Torque steer is their greatest enemy, as this is induced by the difference in driveshaft length from the side-mounted transmission. The angle of incidence of the driveshafts from the differential will be different for either side. This will expose the differences in torsional stiffness due to the differing lengths. The longer shaft will naturally have a lesser torsional stiffness which means it transfers power less efficiently due to twisting more, forcing the car to steer slightly to the side that the longer driveshaft is on.
This niggle has been combated by engineers using multiple ingenious methods. The first is matching the torsional stiffness of the driveshafts by having one hollow and the other solid. This will allow for a more-balanced transmission of torque and was first used by Fiat in its front-wheel drive 127 and by Ford in the original Fiesta.
Another method is equalising the shaft lengths by using an intermediate shaft from the transmission. This means the two shafts can be of equal length, with slack being made up by the intermediate shaft. Low-friction differentials can also be implemented to try to equalise the torque transmission through each shaft.
Transverse engines are also restricted in terms of their displacement and potential power output. The restricted horizontal space in a front-wheel drive, front-engined car means a large engine and transmission isn’t capable of fitting under the bonnet.
This is the very reason that most high performance cars opt for a longitudinally-mounted engine and drivetrain, with only select manufacturers like Noble, Toyota (with the MR2) and Lamborghini (the Miura) opting for a transverse engine mounted in a mid-engined position. The current Honda NSX was originally set to use a transverse V6, before switching to a longitudinal setup.
Longitudinal engines are generally used in rear-wheel drive setups, especially when a large displacement engine is needed. These engines are mounted down the centreline of the vehicle, forming a straight path from the crankshaft to the transmission, propshaft and the rear differential. Few manufacturers have ever opted for a longitudinal front-wheel drive system, but it has been implemented by Audi in its mid-range cars using a transaxle which is also longitudinally mounted.
Things don’t really work in favour of longitudinal engines in terms of pros and cons. The transmission of rotational energy from the crankshaft is less-efficient than a transverse engine as the rotation has to change direction by 90 degrees via a differential to drive the wheels. Cabin space can also be hindered, through the long engine itself effectively pushing the dashboard placement back, and through the transmission tunnel running down the centre of the car.
In defence of a longitudinal setup, the front-to-rear weight distribution of the car as a whole will be preferable to a transverse orientation where mass is accumulated at the front of the chassis. This should make a car more predictable and is advantageous to all-wheel drive vehicles. The in-line nature of these setups also allows manufacturers to implement complex all-wheel drive systems using torsen differentials and viscous couplings directly down the line from the transmission.
In terms of modifications, a longitudinal engine leaves lots of scope for engine swaps. From an in-line four to a grumbling V8, the orientation of the engine block normally gives enough space for a larger powerplant and the lack of driveshafts up front adds to the simplicity in comparison to a transverse swap.
Although we’ve only looked into front-engined setups, there is a huge range of orientations that can influence weight distribution, handling and power delivery. Front-engined, rear-engined, mid-engined, all-wheel drive, front and rear-wheel drive; every setup will have driveshafts, diffs and transmissions in different places which produces a whole list of pros and cons for each.
The argument of transverse vs longitudinal really boils down to the application. Cars that aren’t short of or don’t prioritise interior space - like saloons and sports cars - can easily accommodate a longitudinal setup to take advantage of a large, powerful engine. Meanwhile, the majority of hatchbacks will go for the compact transverse option and are all the better for it. Both do their jobs extremely well and - in this world of impending electrification - long may this argument continue.
Which engine layout do you prefer? Do you feel transverse is inferior to longitudinal, or vice versa? Comment with your thoughts below!