It didn't start with the Vega, but that's one example of open deck design.
A reader writes asking about car repair. The mechanic says his friend's Volvo has a cracked cylinder wall causing misfiring. At first I was skeptical - a far more common (and cheaper) cause of misfire is a bad coil. Modern engines use coil-over plugs, with one coil for each spark plug, eliminating the distributor and instead firing each plug using a computer signal. Cheap and easy to install. But a cracked cylinder? Just not a common thing!
Or is it? I searched online some more and discovered that some Volvo engines are of an "open deck" design and indeed, the cylinders can crack. A long way from the indestructible old Volvo 240 of the 1970s! But this may be an issue caused by open deck design.
I saw a video about this design on YouTube, relating to GM's ill-fated "fuel-pincher" diesel, which was a short-lived (in both senses of the word) engine from Detroit Diesel. It was so bad, GM was forced to sell the division to Penske. One of the issues with the engine was that it was of open deck design. Open deck, as shown above, has no flat surface surrounding the cylinders. The cylinder head acts as the "top" of the cooling jacket. This provides better cooling for the top of the cylinder, but since the pistons are - at some point in the stroke - pushing against the sidewall of the cylinder, it can cause the cylinder to "walk" and bend, which over time can cause fatigue and indeed, cracks.
The Vega engine was famous for this design and today it is used in a variety of applications, usually in relatively low-power vehicles - not race cars or diesels. It is easier to manufacture (die casting, as in the Vega, can be used in place of tricky sand-casting) and again, has some cooling advantages and weight savings. While the Vega engine was ill-fated, much of the technology used in it was adopted by other manufacturers, including BMW, and not without difficulty. The Reynolds high-tectate silica aluminum did give BMW fits when the high-sulfur content gas in the 1990s (in the US only) caused cylinder wall erosion.
But such is the nature of modern engine design - we do things today to save weight, improve efficiency, and increase power. These things often have reliability issues that are not detected until millions of cars are in the field and have over 100,000 miles on them. It is the golden age of internal combustion - but perhaps also the last gasps of a dying technology.
Closed-deck designs are what we think of traditionally.
In the old days, when engine blocks and cylinder heads were made of (sand) cast iron, we were used to seeing the closed block design as shown above. Even a child can see that the open block has less support for the tops of the cylinders, which look more like flower vases than a proper engine block. Aluminum blocks had another issue we weren't familiar with "back in the day" and that was galvanic corrosion. Many engines (again, BMW) had iron blocks and aluminum heads (the reverse of the Vega) and the two metals, when immersed in a liquid solution (say, for example, coolant!) would cause aggressive internal corrosion. Special coolants were needed to reduce this corrosion, but many car owners were blissfully unaware of this requirement until it was too late. Don't get me started on "water wetter" - it reduced surface tension of coolant to prevent cavitation in some diesel engines.
And of course, the different coefficients of thermal expansion of iron and aluminum could cause all shorts of hi-jinks when the engine overheats. Such was the fate of our Vega when my idiot brother ignored the "temp" light.
Like I noted before, they change this technology and don't explicitly tell us. A homeowner puts galvanized decking screws in his new "greenwood" pressure treated deck planks, unaware that this new kind of wood will eat through the screws and bolts within a few years. Hilarity ensues.
Are open-deck engines a bad design? Not necessarily and you may not have a choice in the matter as more and more manufacturers go to designs like this. Try buying a car without variable valve timing these days - it just can't be done! Again, referring to the Vega, aluminum cylinder walls don't wear well, unless you pull some sort of "trick" like GM (and later, BMW) did by using this high-tectate silica which had microscopic bits of silica in the aluminum, which were exposed using an etching process during manufacturing, creating a diamond-hard wear surface. In theory, anyway.
Sleeving engines was another approach - and one done to repair engines where blocks are damaged. On older engines where there is plenty of "meat" left in the block, they can be bored out and steel sleeves driven in (press fit) to create a new cylinder wall. Supposedly, many a Vega engine was "fixed" this way. Others use sleeves from the factory and apparently Volvo is one of these (from what I read and for certain engines). The cracked sleeve can be bored out and a new one driven in. But of course, that costs thousands of dollars - just removing the engine is an all-day job, as is re-installing it. I would think a cheaper solution would be a junkyard engine from a wrecked, low-mileage car. But that's just me.
Should you buy an open-deck engine car? Chances are, you may already own one or have owned one without knowing it. It isn't exactly a car feature that manufacturers crow about in the literature. And there is no real data on whether open-deck designs work better than closed deck designs. Our Kia Soul with the "Theta" engine is a semi-closed deck design with little ribs supporting parts of the cylinders, unlike the "flower vase" design of the Vega. Our "ecoboost" 3.5 in the King Ranch is an open deck design (cranking out over 350 HP from 3.5 liters!). So much for open deck being limited to "low output" engines. Very few "closed deck" designs remain out there as they cost more to make and weigh more. The old "small block" Chevy V-8 is still a closed-deck design (AFAIK) and maybe a few other "old school" engines. Meanwhile, over at BMW, even the "high performance" engines are of open-deck design.
I suspect that, if properly designed, an open-deck engine can be as reliable and long-lived as any other design - closed or semi-closed, for example. However, it seems every vehicle has its own share of esoteric failure modes as they age - few are "bulletproof" as some folks like to claim. Even a Subaru has timing belt issues.
Funny thing is, if you go back historically in time, engines were cast as a block and the cylinders separately (sometimes in pairs) and then bolted to the block. Head gasket materials were somewhat weak back then, so often the cylinder head was cast as one piece with the cylinder itself - which made machining for the valves a little tricky. Some general aviation aircraft engines use this technique, where the "jugs" (cylinders) bolt up to the block. Of course, way back in the day, the "cooling jacket" on a car engine was literally that - a thin brass sheetmetal jacket soldered around the cylinders!
How times have changed. For the better, in most part.
