All-aluminium engines (head and block made of aluminium alloy) are increasingly popular. Mass production all-alloy engines such as Rover K-series, BMW M52 straight-six, Nissan VQ-6, Jaguar AJ-V8, Mercedes V6 / V8, GM LS1and Northstar V8, Peugeot's 2-litre four and GM's new four-cylinder family proved that aluminium block will spread to nearly all cars in the near future.
Aluminium head has been popular much earlier and most engines now employ it. Car makers favour it not really for weight reduction, but for its better cooling property. As 4-valve head generates more heat than 2-valver, aluminium cylinder head seems to be a good solution.
Block went to aluminium much later, mostly because of cost reason. Block is the heaviest part of the engine, thus using aluminium can save dozens of kilogram and benefit a lot to weight distribution of the car. On the other hand, it is also much more expensive, simply because aluminium is pricier than cast iron.
Intake manifolds is another heavy component, especially today's variable length manifolds. Using aluminium alloy instead of cast-iron was just the first step. Many car makers now switched to thermoplastic manifolds made of Nylon 66 or other heat-resisting reinforced plastics. It's cheap, light and free-flowing, nearly a dream for car makers.
However, plastic manifold's biggest flaw is noise, which is considered to be too much for luxurious cars. Therefore Mercedes-Benz chose to use Magnesium manifolds. This material is even lighter than aluminium, although a bit dearer and less resistant to heat. No problem, intake manifold is not too hot. Like any metal, air flow in Magnesium pipes generates less noise than plastic one.
TVR's and Ferrari's
employ Kevlar for intake manifolds.
Whether an engine responsive and high-revving depends very much on the inertia of reciprocating parts, i.e., crankshaft, pistons and connecting rods. While crankshaft material is still bounded to steel for the reason of strength, pistons of high-performance engines are usually made of aluminium. The lighter the pistons, the higher rev and power the engine obtains.
Using alloy pistons is not very costly, what prevent most mass production all-alloy engines from using them is the friction generated between pistons and cylinder walls. It is commonly known that the contact between two aluminium surfaces results in high friction - much higher than between cast-iron and aluminium. Therefore many engines with aluminium block have to employ cast iron pistons.
The most common solution is to insert a thin cast-iron liner to the cylinder, covering the cylinder wall and surround the aluminium piston. Of course, this lift production cost at bit.
An alternative solution was introduced by Chevrolet Vega in the mid-70s. Its Cosworth-designed all-alloy engine employed iron-coated aluminium pistons, thus the block could be linerless. However, it's more expensive than cast-iron liner while not delivering as good performance as Nikasil treatment so that no longer in use today.
Instead of cast iron liner, Nikasil treatment coats a layer of Nickel-silicon carbide, usually by electrolytic deposition, to the inner surface of aluminium cylinders. Since Nikasil layer generates even less friction than cast iron liner, revability and power are both enhanced. Moreover, it is only a few hundreds of a micrometre thick, therefore the spacing between adjacent bores can be reduced considerably, making the engine smaller and lighter. Since the early 70s, Nikasil treatment has been the most favourable solution used by high-performance cars.
The last alternative is fiber-reinforced metal (FRM) cylinder sleeve, which is used by Honda NSX 3.2-litre. Its cost and power / space efficiency are both half way between cast-iron liner and Nikasil. A fiber-based material in the form of cylinder sleeve is first inserted to the die of the block. Melted liquid aluminium is poured into the die and integrate with the fiber sleeve. Then the cylinder wall is machined to the desire bore dimension, leaving only 0.5 mm thickness to the fiber sleeve which covers the cylinder wall. It generates lower friction than iron liner, thus improves rev and power. Moreover, the fiber sleeve reinforces the block, allowing the distance between adjacent bores to be reduced yet maintain mechanical strength.
Everybody knows titanium is light yet strong, although it is very expensive. Finally, this aerospace material spreads to road car use, although still bounded to high-end sports cars. Lamborghini Diablo, Ferrari F355 / 360 M / 550 M etc. and Porsche 911 GT3 use it to raise engine's revability to what would have been impossible.
Forging seems very old-fashion, but there is still no alternative way to obtain high-strength yet lightweight parts without it. From Honda Type R to all exotic supercars, forged pistons, crankshaft and con-rods are commonly used.
Forging is done completely manually, therefore more human-intensive and expensive. Forge the heated metal into a die result in more homogeneous and closer depositioning of metal atoms, thus improved strength and heat-resistivity. With higher strength, the part can be made thinner and lighter, eventually benefiting rev and power.
Forged pistons are also polished by man to further reduce surface friction.