Internal combustion engines have always been constructed from castings - in the last fifty years or so, generally from two major castings - the block and head (or heads). Even the sump is these days often cast, adding structural integrity to the block.
However, this approach has a few disadvantages, especially when it comes to the small engines used in the industrial and commercial applications such as pumps and lawnmowers. The problems? The capital costs of setting up for new castings is considerable; because of dimensional inaccuracies and required surface finishes, lots of a casting is normally machined off; and to take this machining into account and to also overcome problems like porosity, the mass of the castings is relatively high.
However, castings are used in every production engine made today - so is there a better way?
A New Zealand company thinks so. Alu-X Systems Limited, based in Hamilton, has developed an engine that replaces castings with extruded aluminium sections. The ALU-X approach uses interlocking modules, made by cutting extruded profiles to length. (See the "So What's Extruding?" breakout.) Each cylinder and the crankcase are formed from extrusions. Any number of these modules can be joined to make the engine block, replacing one-piece cast blocks and allowing engines of different capacities, cylinder numbers and stoke lengths to be made from basically the same external parts.
The two engine designs that the company have built also use extruded aluminium alloy components for the engine head and sump plate.
The assemblies are held together by a combination of dowels, dovetail joints and through-bolts. T-shaped slots incorporated into the extrusion profile allow simple attachment and adjustment of auxiliary components, in addition to the possible mounting of the engine itself.
Engine Design
Initially the company decided to produce a 200cc capacity engine of about 5kW output - the sort used typically to drive a small pump or generator. In order that off-the-shelf internals could be used, a bore size of 67mm was chosen, with a stroke of 55mm. The main bearings are of the ball bearing type, and - unusually - piston-valves are used instead of normal poppet valves. A piston-valve covers and uncovers a port during its operation - in this engine, two piston-valves in a cross-flow side-valve configuration were used. The piston-valves are driven by two secondary cranks, working in a scotch-yoke arrangement.
Two main extrusions were used to form the engine - a cylinder extrusion and a two-part crankcase extrusion. Both were formed from 6005A aluminium alloy, tempered to T6 standards. (See the "Six-Oh-What?" breakout for more on aluminium alloys.)
Cylinder Extrusion
The block extrusion profile (ie cross-section) is shown here. As can be seen, in addition to the main piston bore, there are also two bores for the piston-valves. Note also the fins on the inside of the cylinder water coolant passages, which aid heat transmission to the coolant. The cylinder walls needed to be strong enough to withstand the in-cylinder pressures and to retain sufficient rigidity - Finite Element Analysis showed that the expected deflection of the cylinder walls was 0.05mm under full combustion pressure, which the company regards as satisfactory.
Both the piston and valve bores were extruded 1mm smaller than the final build spec, to allow for machining to precise size. However, the finished extrusion proved to have a spec within ±0.05mm of design size, and so the machining allowance proved unnecessary. However, honing of the bore may be required in some applications. The surface finish of the bore has been trial treated with both nickel plating and anodising, with both apparently giving good results. Many production cast alloy engines use proprietary treatments of the bores (eg Nikasil), and such a treatment could also be applied to the extruded engine.
The cylinder extrusion provides for the through-bolts. These medium tensile alloy-steel bolts extend from the head to the sump, holding the entire engine module under compression. Alloy steel was chosen in order that the required strength was provided with the bolts still having enough elasticity to cope with the thermal expansion of the alloy extrusions. Four 8mm bolts run through the engine to provide support for the crankshaft bearings, while another four bolts hold down the perimeter of the engine block.
The T-slots are provided in the outside of the cylinder extrusion to allow the easy connection of accessories and/or engine mounting plates. Rather than use drilled-and-tapped holes, the T-slots allow a purpose-designed fitting to be bolted in place - unlike threaded holes, they place the alloy in compression rather than shear and so do not require substantial amounts of material to provide adequate strength. In this engine, the inlet and exhaust manifolds, carburettor, muffler and engine covers are held in place by fittings locking into the T-slots.
Crankcase Extrusion
The crankcase is formed from two sections of identical extrusion. The 'split' of the two parts is along the centreline of the crankshaft, with each section having a semi-circular cut-out to nestle half of the main bearing journals of the crank. As indicated earlier, ball bearings are used - however, the company suggests that plain bearings can replace the ball bearing races if required. The lower half of the crankcase is effectively the sump - if an additional oil capacity is required, the lower extrusion can be simply sectioned as a longer piece!
Head Extrusion
Because of the side valve design, the head is a very simple extrusion - although, as with the cylinder extrusion, it must cope with combustion pressures and temperatures. In this design the underside of the extrusion - the part that will become the top of the combustion chamber - was extruded as a flat piece, with the combustion chamber volume machined into it. Also machined were holes for the sparkplug and the through-bolts.
Not shown here - but included in the extrusion - were passages for the transport of coolant to the cylinders.
Sump and Cover Extrusions
Both the sump cover....
... and the cover plate are very simple extrusions.
The sump seals off the base of the engine, and also provides mounting feet. Small ribs formed into the underside of the sump plate provide some oil cooling. In this engine, splash lubrication was used. The cover plate houses the oil seals around the crankshafts (remember, there are additional ones to drive the piston-valves), improves appearance and provides further mounting points for accessories. Another function of the cover plate is to lock into alignment the cylinder and crankcase extrusions - it does this with the aid of two locating ribs that push into T-slots.
The Four Cylinder
In addition to the single cylinder 200cc engine, Alu-X Systems has also developed a 2-litre four-cylinder extruded engine. The use of an existing crankshaft dictated some of the engine's dimensions, but again a piston-valve approach was taken. Compression is 10:1 and a power output of 100kW was the goal. The engine weighs only 75kg.
As with the single-cylinder engine, the cylinders and a two-part crankcase were made from different extrusions. An extruding machine size limitation of 254mm meant that all extrusions were smaller than this dimension. This resulted in the need for each cylinder to be a separate extrusion - they nestle together with dovetail slots and dowels providing the cylinder-to-cylinder rigidity. Again, through bolts hold the engine together - in this case, 26 bolts are used. Other changes over the single cylinder engine includes the use of twin sparkplugs, dry-sump lubrication and engine management.
For this engine, both the sumps and head plates were machined from solid aluminium block, rather than extruded. This was said to occur for cost reasons - apparently, there would be no technical difficulties in extruding these profiles.
Conclusion
Alu-X Systems claims that the "cost of plant and machinery required to produce extruded engines is less than one tenth of that for casting, with a capacity of engines per hour ten times greater."
In addition, the engine design "is stronger, lighter and around 70% of the height of a conventional engine, with several additional benefits including a superior cooling system made possible by including cooling fins and heat sinks in the extrusions. Working prototypes of a 4 cylinder, 2-litre ALU-X are quieter than a conventional engine and exhibit improved gas flows and a higher power-to-weight ratio."
Journalists who have heard the four-cylinder engine run on the test stand confirm that it is remarkably quiet - primarily because of the absence of a conventional valvetrain.
http://alu-x.com
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So What
Extrusion is a metal-forming process where a big chunk of metal is forced under pressure through a die. It's similar to the way in which cake icing is forced through a pastry pouch to make lines of icing in a particular shape - or how toothpaste forms a round cross-section as it is squeezed out through a round hole. Extruded tubing and hollow shapes are formed by placing a steel mandrel inside the die opening. The aluminium flows between the mandrel and the die, reproducing the shape of the mandrel on the inside of the section and the shape of the die on the outside. OK - you want more detail? The basic material for extruding comprises alloyed aluminium logs. Logs are approximately six metres in length and 130-230mm in diameter. They weigh 700kg each and are cut into smaller lengths - called billets - for use during the extrusion process. Billets are heated up to 450-500 degrees C in a furnace and then forced (at pressures of up to 680 Mpa) through a die with the aperture shape cut into it. The extrusion exits at a speed of 300 - 3000 metres per hour and in lengths of 25-50 metres. On leaving the die, the alloy can be quenched or solution heat-treated via water baths, water sprays or in forced draught air. Following the cooling process, the extrusion is stretched to remove any possible tensions and twists. The product may then be cut to length, despatched, age-hardened and/or sent for surface coating (eg anodising or a print process). Probably the most common household aluminium extrusions are window and sliding door frames.
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Six-Oh-What?
Most extruded aluminium has been alloyed with one or more additives - mainly silicon and magnesium - and is found in the International Standards 6000 series.
- 6005A Alloy - Medium strength alloy, good extrudability can produce reasonable complex shapes due to low quench sensitivity. Mill surface finish fair to good.
- 6063 Alloy - Suitable for intricate extruded sections of mid-strength. High corrosion resistance and a good surface finish. The most widely used alloy. Typical uses include architectural members eg glazing bars and window frames, windscreen sections, road transport.
- 6063 AA Alloy - A stronger version of 6063 but retaining most of that alloy's good surface finish and formability. Typical uses include road and rail transport, general engineering, ladders and light structures.
- 6082 Alloy - The recommended alloy for structural purposes with good strength and general corrosion resistance. Road and rail transport, scaffolding, bridges, cranes and heavy structures.
- 6101A Alloy - The best combination of electrical and mechanical conductor properties with conductivity of 55% of the International Annealed Copper Standard. Typical uses include bus-bar, electrical conductors and fitting.
- 6262 Alloy - Good free machining characteristics, medium strength. Good corrosion resistance and good anodising.
- 6463 Alloy - Based on high purity (99.8%) aluminium, this alloy was developed to respond well to chemical or electrochemical brightening or anodising. It has excellent formability. Typical uses include shower cabinets, picture frames, motor car trim and other applications requiring a bright finish.
The properties of alloys can be improved by heat treatment after extrusion, producing a 'temper'. There are three tempers:
- T4 - solution heat-treated, usually carried out on-line by forced-draught air, water spray or water bath. This is ideal for severe forming applications. Over time some natural ageing or hardening will take place.
- T5 -precipitation treated (artificially aged). This rapidly speeds up the rate of precipitation which means the product will have a very high strength and hardness.
- T6 -solution heat-treated and precipitation treated (fully heat treated). A combination maximises strength and hardness and provides a fully aged material.
More details:
Hydro Aluminium Extrusion Europe
Capral Alumium
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