A carbon fiber engine block is possible, but unlikely

  • Post category:Engenharia

Originally posted on my disabled blog LoneArtisan.com

High-performance machines, nowadays, have an ever-increasing number of carbon fiber parts. However, none of the most important names in the game released a carbon fiber engine block, yet. Carbon fiber engine parts, such as valve covers and manifolds are already a reality, but engine blocks are unheard of. There are some good reasons for that, and they are plenty, in fact.

First, let’s think a while. Why should be a great idea to build a carbon fiber engine block? I think the answer, mostly, has to do with the popular hype of the material. Since every performance part today is made of carbon fiber, an engine block would be amazing, right? Carbon fiber is very strong and lightweight, which means the ratio between power and weight should be heavily improved, at least in theory.

Carbon fiber engine blocks still don’t exist because the material could not handle the thermal requirements, chemicals and the uneven stresses it would be required. The reality is that this matter isn’t so simple as that, and a carbon engine block still doesn’t exist because of a variety of reasons, some of which I will point out below. Of course, this is not an enclosed list, since engineers must have more objections or solutions to those problems. But it is a good starting point for a discussion, especially because in forums people are talking mostly about the temperature limits of the resin.

Heat management

So, as I just pointed out, the first big objection against carbon fiber engine blocks is that the resin may not withstand the operational temperatures of an internal combustion engine. Most resins are able to resist well up to 250°C, so something special should be developed to be used in an engine block. The resin will also face other challenges, such as chemicals, as we will see soon below.

We must have in mind that, as a composite, carbon fiber is composed of two parts, a carbon cloth and a resin. The carbon cloth itself could, in theory, take a lot of heat without much problems. Also since it is mostly a graphite-like material, heat conduction by the fiber is great, but just in their direction. This, in fact, can be used to tailor the heat flow in the engine block, but this property can’t be decoupled from the tensile strength of the part, so heat should flow along with stress.

Also, bear in mind that, even working as a team, the carbon cloth and the resin are very different materials. The resin, for example, has a much smaller heat conductivity coefficient than the fibers, which create a lot of new difficulties for scavenging heat from cylinder walls and oil, for example.

To make matters worse, although the operating temperature of an engine is far below 250°C, this temperature is not uniform, and some spots of the block can become high temperature islands, which would lead to failures. Another problem is that the thermal conductivity, as the tensile strength, is greater in the fiber direction.

So, transferring heat from one area to another is a big challenge, especially because the fiber layers would be laid in different directions to attend to the stress requirements. Adding insult to injury, the embedded resin would, in fact, difficult the heat transfer between fibers, creating isolating layers. Solve the thermal conductivity problem while making the whole thing strong enough is an enormous challenge, and both things cannot be decoupled, since they depend on the direction the electrons travel along with the fiber.

Pistons

This is another concern for an engine block. Although most friction happens in bearings and journals, pistons are a huge concern. And carbon fiber isn’t great where it comes to friction because of the resin. Also, one just can’t machine a carbon fiber cylinder to precise specifications to fit pistons in a bore. Also, cylinders must deal with very high temperatures, much above the 250°C limits of conventional resins, let alone dealing with gasoline and uneven stresses all over the bore at the same time.

However, there might be a solution to that which is using metal cylinder liners. This is not new to engineering, however, since aluminum isn’t a great cylinder material, either. So, engineers are used to developing engines with cylinder liners separated from the engine block. But, the fitting in carbon fiber would be much more problematic. When using a steel bore into an aluminum block, the liner is pressed into the block and held using a method called interference fit. In carbon fiber, these methods don’t work that well, and the liner should be glued using resin.

Also, heat must be extracted from the liners and guided outside of the block through the anti-freeze and cooling system. As opposed to metal blocks, our hypothetical carbon fiber block would have a hard time doing this, as we mentioned above. To add further complication, carbon fiber is a material known for its small thermal expansion coefficient, and that doesn’t match a metal very well. This alone could turn fitting cylinder lines into a nightmare.

Chemicals

Another problem that every internal combustion engine block has to deal with, generally speaking, is the three main chemicals: oil, anti-freeze, and fuel. Every engine block has galleries and passages to distribute oil to all the parts where it is needed. There are some complications here. First, all those galleries must be manufactured somehow, and this is not as simple to do in a lamination process as in a die-cast. The engine block would need to be produced in several steps, and this, of course, increases cost and production time.

The second problem to be faced here is that oil can be an aggressive chemical, especially at higher temperatures, and may damage the resin. As a result, the internal parts can develop cracks and delamination, leading to leaks, cracks or mechanical fails. A special resin, able to stand those chemicals, should be developed. And this adds up with the requirement of withstanding a lot of heat.

Other solutions may come in handy, like using metal tube inserts as oil and coolant galleries or even developing an air-cooled engine. However, those are departures that add too many complications to a technology that is already excellent. Metal engine blocks are doing great for a long time, so why bother that much to substitute them?

Uneven stresses

The most important feature of carbon fiber, of course, is its strength in the direction of the fibers. However, this is not always a desireable feature. In engines, the acting forces are pretty much spread everywhere. For example, while a piston creates a downward force in the crank, it also creates an outward stress in the cylinder walls and an upward force in the head. The engine block is the part which holds everything togheter, so it must withstand all those forces coming from everywhere during normal engine operation.

One of the advantages of metal engine blocks is that they can handle multidirectional forces much better than carbon fiber, as I had already written about in this article, due to the way metal grains bond themselves together to form a part. On the other hand, heat also spreads evenly in metals, so it is way easier to match strength and metal heat conductivity to produce a sufficiently strong block.

Another problem with carbon fiber is that it requires a lot of computational power to solve the problem of stress distribution in the engineering phase. Although engine manufacturers have a lot o incredibly powerful software and computers, their time is precious and used everywhere to design better vehicles. And, since metal engine blocks do their job very well, it may not be economical to build a carbon fiber engine block due to the engineering and computational hard time.

Manufacturing complexity

As I always stress here in the blog, manufacturing is key when fabricating good quality carbon fiber parts. In the case of an engine block, which is indeed a very complex piece, precision is king. And, to overcome all those difficulties I had already pointed out, the block construction would have to be done in several steps. Each step requires incredible precision to guarantee the design requirements are met.

For comparison, this is one reason why metal engine blocks are everywhere. The die-cast method is very powerful, can produce high-quality parts for cheap, and is highly scalable. Industrially speaking, the metal casting is a dream because it allows mass production, quality, and low cost. There is a reason it is around for centuries, right?

There are initiatives for making engine blocks from other materials, such as plastic. But metal is so good and cheap at it that is really hard to find a substitute. And, really, carbon fiber doesn’t look that great of an idea when we face the additional costs in front of what could be gained.

How carbon fiber could be effectively used in an engine block

At least for now, carbon fiber can be an excellent compliment to engine technology, and there may be great uses. Reinforcements, mounts and accessories can help engine performance without sacrificying core strength. However, I doubt we are going to see a carbon fiber engine block any time soon.

To add one more good point against it, thermal engines are hitting their dead-end. Some big players, such as Volkswagen, already announced they will not develop internal combustion engines any further. Instead, they are going to electric vehicles. In this case, there is a far better chance that carbon fiber can be used to fabricate engine cases, for example. But that is a matter for another article.

What do you think? Are we going to see carbon fiber combustion engine blocks any time soon? I would love to read your comments below!

Fábio Ardito

Pelo mundo atrás de treta.

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