This article covers the design considerations for: Cylinder heads, cylinder blocks, sumps, lubrication systems (including oil pumps), superchargers and turbochargers.
/rigid_engine_design.jpg) |
| Fig 1 - Engine with cutaway section to show pistons. |
The engine is the heart of any car, or petrol or diesel driven vehicle for that matter. A poorly designed engine means a poorly designed car, there is so much to consider in the design for an engine that a blog post just won’t cut it. I will however try to cover all of the key areas and explain the basic principles and design considerations for major parts.
Just an initial point, this article doesn’t contain anything on the subject of the power cell unit (piston, crankshaft etc.); that can be found in the piston design 101 article here: LINK
Alternatively you can look in the categories section in the right hand sidebar and click on the ‘101 guides’ or ' section to see my range of articles on vehicle design.
Cylinder Head
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| Fig 2 - Cylinder head (from below). |
Starting from the top is always a good idea so first of all we will look at the cylinder head and work our way down through the engine. The cylinder head is very important; it provides the upper portion of the combustion chamber and houses the valve train and camshafts. Generally speaking the main tasks of the cylinder head are to:
- Form the upper part of the combustion chamber.
- House the valve-train and its lubrication system.
- House spark-plugs (in a petrol engine) and fuel injection system.
- Resist combustion loads and allow cooling for valves, spark plugs and ignition.
- Provide sufficient oil drainage
- Seal and transfer charge whilst minimizing thermodynamic and breathing losses.
As i'm sure you can see already there are a lot of components involved here and it makes this part of the car very difficult to package well, as well as this it is subject to considerable cyclic loading from the combustion process and so needs to be very durable to repetitive wear.
Cylinder heads are typically made of cast iron or aluminium, cast iron has the advantage that it is very strong and cheap, aluminium however is more expensive but is very lightweight in comparison. An aluminium head would typically have the weigh about 50% of the weight of a cast iron head. Cast iron performs acceptably well but it is also more expensive to repair and harder to port (and more expensive to port) than its aluminium counterpart.
There is a future trend in the industry towards using Aluminium based silicon blend alloys as they provide a low mass and a high thermal conductivity to protect it against the thermal loading present from combustion.
The coolant systems housed within the head are often quite complex and require computer based tools to optimize, however after a design has been provided using FEA (finite element analysis) and CFD (computational fluid dynamics) then it is possible to test physically in a lab. This is done by fitting an engine with the prototype coolant system and make the head out of a clear plastic material so you can physically watch the coolant flow around during operation. This technique can be enhanced by dying particles of coolant in fluorescent colours and using high speed cameras to track them, this can help to find areas of blockage and poor circulation.
First generation direct injection used a layout by which air the fuel was wall guided round the cylinder, this lead to efficiency improvements of around 5 - 10%. The aim when injecting fuel and is to cause turbulence in the flow, this is known as forward tumble when done effectively, this does lower volumetric efficiency of the engine but allows for faster and more complete burning of fuel. The first generation system used a wall mounted spark plug to ignite the fuel. In the second generation layout designers managed to package the system so that the fuel and spark plug were both mounted centrally along with the valves, the differences are shown in the diagram below. Second generation injection lead to further efficiency benefits of around 5% and reduced the amount of wall wetting (fuel sticking to and burning on the walls of the cylinder).
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Fig 3 - 1st Generation DI Layout
(Notice side mounted injector). |
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Fig 4 - 2nd Generation DI
(showing forward tumble in red). |
Camshaft
Camshafts are housed within the head and control the timing of the valves, camshafts typically move at half of the engine speed. The most common valve-train layout in modern cars is rows of poppet valves driven by camshafts. A layout called 'roller finger follower' is used in premium gasoline engines, this involves adding a rotating wheel to the rocker arm which controls the valve, this means that instead of rubbing surfaces we now have rolling surfaces so friction is reduced and thus the engine is more efficient. Generally camshafts are made of cast iron or steel. 3 common layout pros and cons are shown below:
Overhead Valve (Pushrod):
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| Fig 5 - Pushrod Valve System |
- Pros: Simple and proven system, easy to implement in side mounted valve engines.
- Cons: High mass and component count, rods restrict port layout, not usable for small powerful engines.
SOHC - Single Overhead Camshaft Layout (one shaft for both exhaust and inlet valves):
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| Fig 6 - Single Overhead Cam Animation |
- Pros: Reduced component count and reciprocating mass, low cost and higher engine speeds possible.
- Cons: Restricted variable valve timing between inlet and exhaust valves.
DOHC - Double Overhead Camshaft layout (one for exhaust one for inlet):
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| Fig 7 - Double Overhead Cam Animation |
- Pros: Same as SOHC plus, higher engine output and reduced emissions
- Cons: More components than SOHC and therefore more expensive
The future of the automotive industry seems to be heading towards variable valve-train systems, many manufacturers are currently working on using these systems to increase engine efficiency.
Cylinder Block
The cylinder block as the name suggests is the block of metal which makes up the cylinders, as such it is subject to significant repetitive loading, from both thermal energy and surface abrasion. 7 key properties that a cylinder block must have are listed below:
- Sufficient thermal conductivity
- Low thermal expansion
- High hot strength
- High strength to weight ratio
- High resistance to surface abrasion
Cylinder blocks tend to be made of cast iron and are bolted down to the end case and the onto the head with long bolts designed to withstand large loads (generally at least 3.5 times the peak gas load from the engine). Designs have been suggested with bolts which travel all the way through the engine assembly keeping all the parts in constant compression, and the load is distributed through a nut plate below the end case. However this is hard to achieve due to packaging issues in a lot of engines.
Honing is used in the cylinders them selves, this is a process by which a number or horizontal holes are cut into the cylinder wall; This enables better distribution oil. Laser honing has also recently been introduced for racing applications and although the mechanisms of honing are not yet completely understood efficiency savings of up to 6% have been claimed.
When it comes to cooling the walls of the cylinders themselves coolant passages are cut in the block to allow coolant to pass around the cylinder and withdraw excess heat. This can be in the form of a wet liner (open topped) which helps to reduce the thermal expansion or closed deck, which seals the passages but means the do not extend the full length of the cylinder so maximum cooling is not achieved. However it means that the block has better structural integrity.
Sump
Most modern cars use a wet sump system for lubrication, this means that the oil supply is housed in the engine right at the bottom, and the oil pump is within the oil pan and it ran directly from the crank; by chain or belt. This case will use a single phase pump, this means that the oil flow is directly related to the engine speed (since it driven by the crank as i just mentioned). A pressure relief valve is also fitted to these systems so that at high load the pressure of the oil entering the engine is not to high. This is a simple piston normally, which will be spring loaded and sat in the oil feed channel, as the pressure increases the piston moves back and opens a hole which allows oil to exit and reduce the pressure.
Testing of lubrication systems is often performed on a what is known as a 'rock and roll rig' as the name suggests this provides a dynamic platform to move the engine around on to check lubrication is sufficient at a number of different angles (that may be typical in situations such as hill climbing or descent). Plastic covers are applied to the engine and cameras are used to observe the flow of oil and check for potential bad circulation or vortices (very bad!).
Dry sump systems have a fairly different layout, in this case the oil reservoir is not housed within the engine itself but in an external tank somewhere else in the vehicle. This requires a multi-stage pump which is externally mounted. This system has a huge advantage in that it significantly reduces the height of the engine. This means that the engine can be mounted lower and subsequently the centre of gravity of the car is lowered. As well as this it can lead to an increase in engine output (since no windage losses) it also means oil handling issues are reduced since it is housed externally. On the down side it is quite a complex system, as well as heavy and expensive.
Superchargers
Supercharging is popular because it means you can have increased performance for a fixed engine size, or equally you can decrease engine size for a given performance. By adding different degrees of supercharging manufacturers can create a range of different output engines from the same base engine design which is obviously very attractive from a manufacturing costs perspective. For those who don't understand supercharging or turbocharging you are basically supplying the engine with more air, which means you can also supply more fuel and therefore get more combustion and more power.
Since you are increasing the power and output of and engine when you supercharge it you need to consider a number of upgrades to ensure the engine can survive the increased loads present:
- Exhaust valve-train design and materials must be revised.
- Reduced compression ratios will be needed since thermal and pressure loads both rise.
- New bearing materials may be needed to withstand the higher loads.
- Increased oil pump capacity is required.
- The breather gas system may need to be redesigned to cope with higher pressures.
Superchargers are often associated with parasitic losses within engines but that can be overcome with bypassing or clutching. In comparison to a turbocharger a supercharger has a favourable back pressure.
There are two main types of supercharger compressor:
Roots Blower Type
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| Fig 8 - Roots Blower Compressor Animation. |
Typically has a peak efficiency of 65 - 70% and a peak compression ratio of 2 and runs at speed of around 20,000 revolutions per minute. It is commonly found in Mercedes, Audi, Mini and Jaguar superchargers.
Screw Type
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| Fig 9 - Screw Type Compressor Animation. |
Generally has a peak efficiency of 75 - 80% and a slightly increased compression ratio of 2.3 compared to the roots blower compressor. However it has a more limited use only found in AMG Mercedes.
Turbochargers
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| Fig 10 - Turbocharger Diagram |
Formula one is currently considering re-introducing turbochargers in 2013 and the BTCC are confirmed to be introducing them this year (2011). A turbo is a compressor which does very much the same job as a supercharger but the difference is in how they are powered. A turbo is driven by the exhaust gases as they are expelled from the engine, they pass through a turbine, which are attached by a single common shaft to a compressor. The exhaust gases spin the turbo up to incredible speeds of around 150,000 to 220,000 rpm with the smallest turbos providing the fastest speeds; thats over 10 times faster than typical supercharger speeds. They have a typical pressure ratio of 2.5-3.0 and the peaks of thermal efficiency lie in between 75 and 80%. Due to these speeds and pressures the air exiting the turbo can be around 140 degrees centigrade and such need cooling, so an intercooler is added which reduced this to around 40 -50. The cooled air is then passed through a throttle which controls the volume flow rate of air which then enters the intake manifold.
Typical materials for various turbocharger components:
- Compressor wheel - precision cast Aluminium alloys
- Compressor housing - die cast Aluminium alloys
- Turbine wheel - Nickel super alloy (Inconel)
- Turbine housing - die cast Iron
- Turbocharger shaft - forged Steel
Turbochargers are FAR more efficient at high speed and high load. At low speeds and loads 'turbo lag' tends to be present whereby very little happens when you press the accelerator since the turbo is not spinning up to speed and as such acceleration can be very poor while the turbo spins up and then suddenly gets up to speed and the vehicle will begin to accelerate quickly.
