The 'Boxer' engine is so called because the movement of the engine's pistons resemble the movement of a boxer's fists in the horizontal plane. However, unlike the boxer's fists that both move in the same direction, half of the 'Boxer' engine's pistons move in the opposite direction.
This very important difference provides the key benefits of the horizontally-opposed (Boxer) engine layout where half of the total number of cylinders and therefore pistons lay on their sides in an east-west configuration moving in opposite directions.
The key benefit of the Subaru 'Boxer' engine's horizontally-opposed layout is not about its power output or environmental performance but its contribution to the vehicle's handling, stability and as a result, safety.
Firstly, because the cylinders in the Boxer engine lay on their sides the overall height of the engine is significantly lower especially when compared to the more normal 'inline' engine where all the pistons move in the same direction in a north-south configuration. This results in a low centre of gravity that has very significant advantages in terms of the vehicle's handling and stability, by helping to keep the tyres more firmly planted on the road surface as a result of a lower level of weight transfer from the inside wheel to the outside wheel when cornering.
Furthermore, because half of the total number of cylinders is on one side and the other half on the opposite side with a common crankshaft sandwiched in the middle the left-right weight distribution with the engine positioned on the centre line of the vehicle is equal. This also makes a very significant contribution to vehicle balance, stability and handling particularly when cornering or making rapid changes of direction.
The overall length of the Subaru Boxer engine is comparatively short compared to a conventional 'in line' or 'V' engine layout as a result of the degree of cylinder overlap that is possible due to the opposed piston configuration. This contributes to an improvement in vehicle front-rear weight distribution that again is a significant factor in vehicle handling, stability and also steering response. If a vehicle's weight is more centrally positioned the forces required to change direction are less due to a lower level of weight positioned in front of the steering axle. The result is more responsive and precise steering, again a key safety attribute.
As mentioned previously because the cylinders lay on their sides positioned in opposite directions with a common crankshaft, the size and therefore the weight of the Boxer engine is relatively lightweight. Subaru 'Boxer' engines also feature all aluminium construction of the crankcase and cylinder heads that also significantly contributes to weight reduction. This not only improves the power to weight ratio and therefore driving and environmental performance but also again helps handling and stability.
Because the pistons move in opposite directions the natural rotational balance of the engine is very good due to the cancellation of the piston inertia forces that move in opposite directions. This means two things;
Good rotational balance means lower levels of vibration and harshness, which translates into less wear and tear and therefore higher levels of durability meaning a more reliable and lower cost of ownership.
Because the crankshaft is sandwiched between two very rigid crankcase halves it is very well supported and therefore its durability is significantly enhanced and because it absorbs less vibration its design can be of a much lighter construction. This not only contributes to the over lightness of the engine but also means a freer revving more responsive engine, both attributes bringing improvements in vehicle active safety through improved driveability.
Another key benefit of the Subaru 'Boxer' engine design is that it could save your life! This is the result of its low height when positioned in the front engine bay. In a full frontal or even an offset accident a severe impact will cause the front of the vehicle to crumple and subsequently cause the engine to be driven backwards towards the passenger compartment with potential penetration and severe trauma to the front occupants. Due to its low height, the 'Boxer' engine can be redirected under the floor of the passenger compartment avoiding any penetration of the passenger compartment and therefore reducing the chance of front occupant injury.
What are the negative aspects of the 'Boxer' horizontally opposed engine design? Why do all manufacturers not adopt this format if it has so many advantages?
Traditionally one of the negative aspects of 'Boxer' engine design was that as the demand for bigger capacity engines increased the only way to achieve a bigger engine was through an increase in the cylinder bore (diameter) size. This was because any increase in the length of the cylinder (stroke) would make the overall engine width greater. This meant difficulty in accommodating the wider engine within the chassis without making the overall width of the car greater, which is not desirable from a packaging point of view and given our traffic and road conditions.
When an engine becomes 'over square' meaning the bore is greater than the stroke, the efficiency of combustion becomes increasingly difficult. Given the minimal time that is available in a relatively high revving engine on each cycle to completely burn all of the fuel, if the bore size becomes too large the flame travel time from the spark plug to the outside of the cylinder wall becomes problematic. The result is that the fuel is incompletely burnt and fuel consumption deteriorates. Improvements in combustion chamber design and computer control of the air-fuel ratio, ignition and valve timing has been able to successfully manage this slightly negative attribute of the 'Boxer' engine design.
However, the launch of the third generation Subaru 'Boxer' engine has seen the introduction of a new innovative design that has allowed an increase in the engine stroke and reduction of the bore size, without increase in the overall engine width, has overcome this previously negative design feature of the 'Boxer' engine.
The other negative aspect of the 'Boxer' engine design is its relatively complex shape that causes an increase in manufacturing costs. In this case Subaru considers the positives outweigh the negative.