New Mazda Turbo Diesel Engine

The first of the sequential turbos is a tiny hairdryer that you could spool with a sneeze. - Brendan McAleer on The Truth About Cars

When did spool become a verb? Possibly when turbochargers first became popular. What makes a turbocharger so great is that is has no mechanical connection to the engine. Oh, there are hoses and pipes and whatnot, but there are no shafts or gears or belts or pulleys of the sort you get with a conventional supercharger. Turbocharger installations can be big and complex, but the bulk of it just sits there, there are no moving parts other than the "spool", which really isn't a spool at all (and air, which really isn't a part). All I can figure is that it bears a very vague resemblance to a spool in that it is a round device that is wide on the ends and narrow in the middle. It is basically two fans, or impellers, mounted back to back on a short shaft with a bearing in the center. It works by using one impeller, driven by exhaust gas, to turn the other, which blows air into the engine. In order to boost the power output of the engine, the impeller needs to be big enough to overstuff the engine when it is running at full speed.

Hard Working Turbo

To do this it needs to be of some size, like a couple-three inches in diameter, and because it is made of metal (in order not to melt from the hot exhaust gases) it weighs a couple of pounds.  Start blowing on a fan and it starts turning. Keep blowing and it starts turning faster. Getting the entire mass of impeller spinning fast enough to do its job can take a while, seconds even. This is what is called "spooling up", or if you are impatient, turbo lag. Same difference. Once everybody gets up to speed it works like a champ. So a turbocharger "that you could spool with a sneeze" must be very small and very light weight.

This a sequential turbo setup that is going in Mazda's LeMans race car. You aren't going to spool these with a sneeze, but you can see the impeller blades inside the inlet of the lower turbo.

We're talking about a new, little diesel engine from Mazda. They have gone to a great deal of trouble to make this engine behave in a civilized manner, i.e. more like a gasoline engine. One of the key points is that they have lowered the compression ratio, and, well, I'll let Mazda tell you:

There are two main problems that have been preventing the spread of low-compression-ratio diesels regardless of these merits. The first is the fact that when the compression pressure is reduced, the compression temperature during cold operation is too low to cause combustion, preventing engine-start. The second is the occurrence of misfiring during warm-up operation due to lack of compression temperature and pressure. 

 

The newly adopted multi-hole piezo injectors allow for a wide variety of injection patterns. Precision in injection amount and timing increases the accuracy of mixture concentration control, ensuring cold-start capability. Hardware-wise, the injector is a high-spec type capable of a maximum of 9 injections per combustion. Along with the three basic injections: pre-injection, main injection, and post-injection, different injection patterns will be set according to driving conditions. Definite engine-start even with a low compression ratio is attributable to this precise injection control and also the adoption of ceramic glow plugs.

9 injections per combustion!?! We're getting into the range where you would actually need those Megahertz clock speeds on the CPU in the engine controller.

Inspired by Dustbury.

Update February 2017 replaced missing text images.