Engine blocks are the unsung heroes of any Engine, but what do they do and how are they made?

1. The Challenge

An engine block is a metal structure which is essentially the ‘rib cage’ of the engine and houses some of the key components such as the cylinders and water jackets. The engine block has to endure the most brutal temperatures and stresses found within a vehicle due to the extreme nature of the combustion process. In the extravagant world of F1, this harsh environment is intensified even further. Within a blink of an eye, an F1 engine completes 200 ignitions, with instantaneous gas temperatures reaching 2,600°C and the consequent pressure forces equivalent to the weight of 4 elephants acting on each piston.

The recent shift towards small capacity turbocharged engines that we have seen in F1 and other categories has further increased the stresses and temperatures within the engine; with the latter the most problematic. Materials suitable a few years ago are now no longer strong enough at these immense temperatures, and the consequent cooling requirements has increased the complexity of the internal shapes for the water jackets; making traditional casting and tooling methods impossible to use.

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2. The Manufacture

Overcoming these Manufacturing limitations required not only establishing innovative technologies, but also ensuring that they could be achieved on a practical level. Traditionally, engine blocks are cast out of metal, however Grainger and Worrall developed ‘sand printing’ which revolutionised this casting process and is used to manufacture engine blocks and components for the likes of F1, LMP1, LMP2, WRC, WRX and Moto GP.

“Sand printing gives us an almost infinite capability to derive shape and size, with minimal constraints,” explains Keith Denholm, Engineering and Technology Director at Grainger and Worrall. “Both Motorsport and Automotive are adding levels of complexity in terms of the shapes, physical and mechanical performance and sand printing does a particularly good job of allowing us to optimise that.”

Sand printing is similar to 3D printing; layers of 0.25mm thick sand are ‘printed’ onto a jobbox with a layer of chemical binder in between. In this way, complex 3D shapes are gradually created, slice by slice. You may be wondering how this helps the manufacture of a metallic engine block; well, these intricate sand printed shapes are called ‘cores’ and are secured within the molds of the casting. Once the molten metal is poured in and has solidified, the sand is shaken out; leaving the complex array of holes and passageways required for a high performance engine. Although sand printing is a relatively simple concept, to achieve the highest quality (which is critical for such a stressed part), an excessive amount of engineering and scientific expertise is applied throughout each stage of the process…

2.1 Stage 1: The Virtual World

The first step is to generate a 3D CAD model and like most components in motorsport this is a battle between the designers who want their optimised shape and the Manufactures who want a design they can actually produce. At this stage, simulations are also used to analyse the behaviour of both the molds and the cores when in contact with liquid metal.

The capabilities of sand printing allow complete design freedom right from the start because many physical constraints are removed. “We can now make the ship in the bottle, which we couldn’t before,” Denholm highlights.

2.2 Stage 2: Molding

Once the virtual design has been finalised, the engineers have the difficult job of thinking inside-out because to manufacture a casting, you need to manufacture the parts that aren’t there – the cores, and this is where sand printing comes in. “We have two printers that produce sand in a similar mechanical way, but have very different chemical systems,” explains Denholm. “The first is a cold curing process, where the binder fixes the layers of sand at ambient temperature, as they are printed. Therefore, once the part is finished, it is already glazed which makes it robust and suitable for large molds. However, for the more intricate cores we need a stiffer, more accurate sand and so we use a hot curing process. Here, the infrared lamp in the printer heats the layers of binder in between the sand to initiate the curing process and evaporate any moisture, before the parts are placed in a microwave for a final cure.”

The sand itself has a tough job, as it not only has to be strong enough to withstand the thermal loads of 700°C liquid metal, but also be weak enough to be shaken out of the mold. “When in contact with the molten metal, the sand will want to expand by approximately 1%, which is not dimensionally accurate,” highlights Denholm. “This is why we not only have several types of sand with different chemistries, but also different curing mechanisms as well.”

Let’s take the example of a cooling jacket which sits between two cylinder bores. The thinnest section of this part is 1.8mm and with a conventional grain of sand measuring 0.2mm, only 9 grains of sand can be used in this cross section. Not only is this weak, but the liquid metal could penetrate between these grains; creating a blockage in this cooling jacket when the sand is removed. Therefore, a part synthetic sand, at 0.1mm is used in the hot curing process to increase the amount of grains packed into these thinner cross sections.

By compacting sand, the cores provide structure for themselves, so unlike other additive manufacturing processes, there is no need to print additional supports. However, other structural features may be necessary to help secure the cores within the mold. “Technically, we can produce a monoblock of sand, which replaces several cores, but you would never do that from a manufacturing standpoint,” explains Denholm. “Firstly, how can you be sure that everything is right and that all the powdered sand is removed? Also, when the metal is poured in, the air has to displace out and we want the feed metal in, so we don’t want it to be hermetically sealed. There are obvious benefits to a single core as the loads are more uniformly distributed as opposed to gluing an assembly together. However, we might aim to make fewer cores, but never just one lump of sand as that’s not the end goal.”

2.3 Stage 3: Pouring Metal

This seems like a simple stage, however during pouring, the metal can exceed critical velocities which induce turbulence and significantly reduce the metal quality. Therefore, precisely engineered gating systems are used to manage the mass flow rate as the metal fills the mold.

“We fill molds uphill because if you pour from the top, the metal will cascade down, similar to a shower, which has a much larger surface area exposed to air then if you were to fill a bath up through the plughole. The latter will expose the water to the area of the bath, roughly a square metre. If you drop that amount of water in through droplets in a shower, the combined surface area could be as large as a tennis court,” explains Denholm. Minimising the contact of aluminium with air is essential to avoid aluminium oxide which is a ceramic and doesn’t weld together with the metal. This can effectively lead to different materials distributed in the casting and so thermal and mechanical stresses are not transferred which can be the basis of fatigue failure.

The secret to achieving a high performance casting is to use the highest quality metal. However, this is impossible because every processing stage throughout the metals life cycle reduces the quality, and introduces the potential of impurities. Similar to fresh food from a Supermarket, which loses flavour every time it is processed from the fields to your plate.

“The very presence of an atmosphere causes all manner of issues for us when working with metals. The metal starts life as ingot and although it has already been processed many times, here it is potentially at its highest quality, but not perfect,” highlights Denholm. “It’s like any natural process in the world, you have this entropy effect where you go from a state of order to a state of less order. But we know that, so we have to ensure that at each stage we minimise that quality loss as much as possible.”

2.4 Stage 4: Solidification

The rate and distribution of solidification can be manipulated to suit the performance requirements of specific areas of the casting. Theoretically, molten metal solidifies by transferring heat to its surroundings, which in most cases is the sand. If the sand was inert and thermally inactive, the metal would stay liquid forever. Naturally, the rate at which the heat conducts from the metal depends on the surrounding media. Therefore, areas of the casting can either be insulated to keep the metal in its liquid state, or placed next to a heat sink, which has a high heat capacity (usually iron or steel) and conducts heat away quickly (coolers). This is how Grainger and Worrall can precisely control the growth of the crystalline structure as the metal transitions from liquid to solid.

“Unfortunately, this process doesn’t happen instantaneously, it’s like the growth of a snowflake,” highlights Denholm. “Take the gas face of a cylinder head where the explosion happens. This is typically an area where fatigue is most likely to occur and so we need to solidify that first to initiate a tighter microstructure with smaller grains. Therefore, we use coolers because the metal will have less time to grow before it solidifies. If you stop a snowflake from growing, it will remain small, which is why on cold snowy days the snow is more like frost, whereas on warmer days you get much bigger snowflakes.” The part also needs to solidify in a consistent way. However, the varying thicknesses of the part disrupts this uniformity, and so coolers are also used to mitigate this.

Once solidified, the metal contracts volumetrically by approximately 7%. To account for this significant change in size, insulated tubes, or feeders are placed on top of the mold and retain the aluminium in a liquid state for as long as possible to continuously fill the voids generated by this contraction. “We’ve got this casting that wants to contract, but is restricted by the molds and the cores,” explains Denholm. “So it starts to react to that and generate residual stress. We work very hard to reduce this stress but it’s impossible to remove it completely. In fact, sometimes we deliberately manufacture parts not to be straight because we know that during solidification, the part will straighten itself.”

2.5 Stage 5: Post Processing

Once the part has been cast and it has gone through a series of machining and heat treatments, the analysis begins. Most parts go straight into the CT scanner, where a beam of X rays is passed through the part and a line detector builds up an image in mm slices. This 20GB set of data is then imported into a software program which reconstructs the images using 250 million greyscales to determine the solid sections and ultimately generates a 3D model of the actual part. This is then overlaid with the initial CAD model, highlighting any areas of variation.

Grainger and Worrall also have optical scanning systems which analyse the surface measurements of parts and build up models, although this cannot ‘see’ the inside of the part. However, once calibrated, these optical scanners use a pre-set program and analyse the quality of production volumes with no need for an operator.

“Usually, the first batch of parts are completely usable, but we may decide to implement minor adjustments of 0.5mm or 0.25mm to our tooling. By using our CT and optical scanners we can continue this iterative process so that by the 2nd or 3rd batch, the products have reached the fully adjusted condition, explains Phil Ward, Director of Performance Products at Grainger and Worrall. “Ten years ago to make a casting, it would take 7 weeks to make the tool, then there was a long validation process of the first sample part and only then could you start manufacture. Now, we can receive a modified design from an F1 customer on a Thursday, use our printed sand processes to cast the parts, inspect them using our new CT technology within 3 hours and supply race grade parts the following Tuesday. It’s an extreme example, but it means our motorsport customers can introduce developments almost weekly which is a radical step from the past.”