The drawbacks of front-mounted engines in modern main battle tanks

The Merkava series of main battle tanks (MBTs) is currently the only real MBT with a front-mounted engine. On light tanks, armored personnel carriers (APCs), self-propelled guns (SPGs) and infantry fighting vehicles (IFVs) placing the engine infront of the crew compartment is a common design practice, but on main battle tanks this is a rarity.

The Merkava tanks with their front-mounted powerpacks are a modern oddity and rarity

The reason for this is that having a front-mounted engine comes with a rather huge number of drawbacks, compared to only a few advantages, which by most countries are not considered as mandatory improvments for a tank.

A front-mounted engine does have a number of drawbacks in regards to the tank's armor protection:

• A frontal engine reduces the space available for special armor  
• Modern composite armor is very bulky and requires a lot of space (more than 600 milimetres for the hull on modern tanks) in order to deal with all available threats; a powerpack is taking up a lot more length (about 1500 mm for the MT 883 engine), but increasing the length of the tank's hull is not desireable, as it has a huge number of drawbacks. In case of the Merkava tank, the frontal hull armor is thinner than that of a comparable MBT from another country. 
• It also increases hull height
• The height required for a seated driver (in a reclining position) is lower than the height required for a powerpack. In case of the German Leopard 2 MBT, the height at the start of the UFP (upper front plate) is about 1 metre above the ground. The height at the end of the UFP is about 1.522 metres above the ground, where the rear section of the hull where the powerpack is mounted, is located 1.774 metres above the ground: mounting the engine at the hull front would increase the height of the hull by 222 mm (or 272 mm if we include the difference in ground clearance between Leopard 2 hull front and rear). That's about half the size of the UFP!
• This also can lead to an increased turret height
• Due to the higher hull and the turret ring being moved backwards (in comparison to other tanks), the turret has to be higher, unless a lower range for gun depression and elevation is deemed as acceptable. If the turret was not taller, the gun would hit the UFP everytime the crew tries to depress the gun. In case of the Merkava reducing the gun depression was chosen, so that the Mark IV has only 7° gun depression instead of the approximately 10° reached by other tanks with manned turrets.   
• A front-mounted engines also means that more weight and volume of the special armor has to be utilized for reaching the same amount lateral protection
• In order to reach what is considered by tank manufacturers a decent level of protection for the crew, heavy ballistic skirts (with a thickness of 65 to 200 mm) are used on the frontal sections of the hull. Unlike the non-ballistic skirts, the heavy ballistic skirts consist of composite armor or ERA, and are designed to provide protection along the frontal 60° arc for the whole crew compartment. Due to placing the crew compartment of the tank behind the engine, a larger area of the sides needs to be protected by heavy ballistic skirts, which means (for a constant weight) that less armor can be utilized for the front.

Additional length for the heavy ballistic skirts (red) and base armor (yellow) are needed on the Merkava to protect the crew (teal)

Compared to a Leopard 2, the Merkava has about 100 mm less ground clearance and an about 200 mm greater height to the turret roof. The height to the top of the UFP is about 300 mm larger on the Merkava, as measured on different photographs of the Merkava II and Merkava III.

This drawing of a M1 Abrams hull shows how the hull front is not as tall as the rear and thus the frontal profile can be kept smaller

While a powerpack and also fuel tanks will offer some amount of protection against impacting projectiles (although less than actual armor will - both per thickness and per weight), it should be taken into account that this is a different type of protection:
Once the powerpack is damaged from a projectile or the fuel systems are ignited, the tank will become imobile and won't be able to participate in any further combat actions. Instead of the crew dying, the damaged tank will be a mobility kill or a mission kill. This also means however that other parts of the forces have to secure the damaged tank and guard it until reinforcements or combat engineers have arrived - because otherwise the damaged tank will turn very easily into a total loss, when enemy forces attack it. In other words: a front-mounted engine can only provide crew protection, the tank as a system however will be even more susceptible to damage.
Actual armor on the other hand does not only crew protection, but also system protection. If an impacting projectile is stopped by the special armor, it won't be able to damage the internal components of the tank. The tank as a system stays intact and can still participate in combat.

The lower front plate of the Merkava is protected by very thin armor only

Furthermore a number of negative performance features are interwoven with the decision to utilize a front-mounted powerpack in a modern tank. The driver's vision in close proximity is reduced, because he is located further away from the frontal edge of the hull and because the hull is taller. The static track tension will be higher. The drive sprokets located at the front are more exposed to rocks and other obstacles, which means at higher speeds the drive sprockets can be hit and will be damaged easier. Also the air-intakes and/or the exhaust vents have to be located at the sides of the hull or at the front, which will get clogged on dusty/sandy terrain more easily. 

In case of infantry fighting vehicles and armored personnel carries, placing the powerpack in the front of the vehicle does offer a great benefit: a rear ramp/door - the infantry squad can enter and disembark from the vehicle without being exposed to enemy fire, while the thicker frontal armor of the vehicle can face the enemy. As there are currently not many purpose-built light tanks, most light tank designs are based on IFVs in order to keep costs down. Here placing the engine at the front means just saving costs compared to redesigning the hull and vehicle.

Like most modern light tanks, the CV90-120 just mounts a turret on the IFV chassis

For self-propelled guns placing the engine in the front of the vehicle is benefical, because the vehicle have a huge demand for ammunition (so a rear door for ammo replenishment is needed) and due to the extreme length of the gun barrels, which requires the turret to be placed further away from the front to reduce gun overhang and add stability while firing.

For the Merkava series the front-mounted engine made sense, when we look at it's history. Israel lacked modern armor technology and thus relied on spaced armor only. The armor layout of the Merkava I and II is optimized for hull-down combat from static positions, which was the most common type of operation for tanks during most of the Israel-Arab conflicts of the past, such as the Yom-Kippur War and the Six Days War. The weaker lower hull of the Merkava would be hidden behind the terrain or prepared (concrete reinforced) positions, so that the hitting them would not be possible. The lack of (bulky) composite armor also meant that no system protection is lost compared to a tank with homogenous or spaced armor and rear-mounted engine.

The frontal hull armor of a Merkava I or II tank. All armor is placed in front of the engine!

The later models of the Merkava series however suffer unnecessary weaknesses from their inheritance. At a 60-65 metric tons weight, but a larger physical size and a much larger armored surface (more armor required for the frontal surface, for the sides, the rear, the roof, aswell as for the mine-protection), the Merkava tanks should not be expected to be as well armored as their European or some of their Asian competitors.

Object 299 prototype tank - the front-mounted engine had a huge impact on mobility and maximum armor thickness.

All major tank building nations have experimented with tanks, which had their engines mounted in the front or in the center section of the tank. None of these experiments resulted in the adoption or creation of a new MBT with front-mounted powerpack.