Liner cooling the OEM wayTags : engine-structure
Although I couldn't possibly guess at the true statistics, I would imagine that at least 99% of all race engines are derived from mass-produced, OEM units. In essence that means modified or 'improved' but nevertheless 'stock' cylinder heads, and perhaps more critically, 'stock' or possibly even standard, unmodified cylinder blocks. The technology of the OEM cylinder liner and its cooling are therefore of great interest if they provide a signpost to the race engines of the future.
When combustion loads permit, aluminium alloy is the material of choice for the cylinder block, since as in motorsport, weight is a critical parameter in the modern road-going passenger car. But unlike race engines, where ceramic coatings on aluminium bores are still preferred, road transport OEM products often prefer cast-iron liners.
For best durability and minimum costs, as well as other factors unrelated to motorsport (such as cold-start emissions), these may be cast on the factory floor during the block manufacturing process, and under normal use should give more than 150,000 miles of trouble-free motoring. But as in motorsport, OE engine manufacturers are downsizing and adding turbochargers, and with space limited in the engine bay, to make engines more compact the spacing between the cylinder bores (sometimes referred to as the bore pitch) is getting less, to the exclusion of the coolant, particularly towards the top of the liner.
One solution is to have what is called 'open deck' designs. Here the liners, located towards their lower end in the cylinder block casting, are effectively clamped against the cylinder head fire face and surrounded with coolant. While excellent for cooling, liners will shuffle under the high gas loadings of a race engine, to the detriment of the head gasket. The alternative is to have a 'closed deck' design, which anchors both the top and bottom of the liner, but doing so limits the cooling at the all too critical position at the 'bridge' between the bores.
In some designs, holes can be conveniently cross-drilled through the bridge area to improve the coolant flow across the block and keep inter-bore metal temperatures to acceptable levels. This is fine if you are machining only a handful of components but for OEM-type volumes, long drills and the risks associated with the drill bit 'wandering' during the process make such measures unpopular. And anyway, these small holes may be easily clogged or difficult to get close to the top deck.
Casting small slots between the bores is perhaps the only elegant solution, but doing so using sand cores is not really practical either. Replacing the cores with small oval glass tubes - something like 4 mm long by 1.3 mm wide but only 0.2 mm thick - is a technique that has been used for small-volume castings in the past but has now been perfected for volume manufacture by at least one OEM. Embedded in the sand cores, the glass is blown out using high-pressure compressed air once the casting has cooled.
Better coolant flow at lower coolant pressures, lower 'bridge' temperatures and reduced thermal gradients all sounds like a good place to start for the next generation of OEM-derived race engines.
Fig. 1 - Glass tube cross-section
Written by John Coxon