READING TURBO COMPRESSOR MAPS, PART IV: EFFICIENCY ISLANDS

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft Forum turbo blog)

"Efficiency is doing things right." -- Peter Drucker

Anyone who has mistakenly put their hand on the shop compressor discharge line will always remember that compressing air results in heat. And NASCAR TV viewers often hear about "starting on low tire pressures" to avoid "pressure build-up."

Both of these are examples of the relationship between air volume, air pressure and air temperature.

Ideal gas law states that:

(air pressure x air volume)/air temperature = remains constant (Miller 19-20) Thus, increases in air pressure of a fixed quantiy of air result in increases in temperature.

When any compressor takes a "gulp" of air and compresses it into a smaller space, heat is necessarily produced. Often, the temp increase is explained as the result of increased friction between air moleules rubbing together when jammed into a smaller space.

Engineers and scientists describe the ideal temperature rise from air compression as "adibatic" -- neither gaining or losing any heat (beyond what the Ideal Gas Law predicts, that is).

However, no air compressor is 100% efficient. Internal air movement, impeller friction, pumping losses, and other inefficiencies add extra heat to the compressed air. This inefficiency is represented on turbo compressor maps and is critical to determining the actual density of the compressed charge.

Looking at the old T66 turbo map, the lines in the middle of the map show zones or "islands" of efficiency. The percentages shown are the calculated efficiency of the compressor, based on measurement of the discharge temperature of the compressed air.

Thus, when someone reports that a particular compressor is operating in the 75% efficient zone at a particular pressure ratio, they're saying that the compressor is heating the air 25% MORE than the Ideal Gas Law adibatic temperature rise formula predicts.

By way of comparison, when a traditional Roots blower is operating in a 50% efficiency island, then it is heating the air 50% more than the ideal temp rise formula predicts.

Extra heat in the compressor discharge air indicates two things. First, the extra heaing means that the dischared air is less dense than under ideal conditions. Second, the extra heat shows that not all of the "work" applied to compressing the air actually resulted in air compression -- some of the energy was "lost" to heating the compressed air. (engineers and scientists refer to this as "isentropic efficiency.")

However, for Car Crafters, the more important things are to determine how much density has been lost and how to recover as much of it as possible.

READING TURBO COMPRESSOR MAPS, PART III

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft forum turbo blog)

"One must strike the right balance between speed and quality."-- The Right Honourable Clare Short (Former MP,Birmingham Ladywood, UK)

The next compressor map elements of note are the series of r.p.m speed lines arcing from the surge limit line on the left over to the right.

Each of these speed lines is a graph of compressor performance at specific compressor r.p.m.

 

Warner (Street Turbocharging) and Hartman (Turbocharging Performance Handbook) both provide detailed explanations of how turbocharger engineers use these speed lines in building a compressor map.

The short version is that the turbos are run up to a particular test r.p.m. on a "test rig" (sort of a turbo dynomometer) and then the outlet flow is restricted with a valve to measure how efficient the compressors are at various mass air flow levels (the bottom axis of the map).

Note that until the compressor becomes a flow restriction (the right side of the map) that pressure ratio and compressor speed are closely linked. The speed lines turn down sharply at the right side of the map because the compressor is simply too small to efficiently supply any more mass air, reducing the pressure ratio.

On this map, the downturn in the speed lines becomes more severe at higher compressor r.p.m. levels. This suggests that increases in mass air flow beyond the efficiency range of the compressor can lead to drastic increases in speed as the compressor struggles to "keep up" with air demands.

The map speed lines also show that for each level of mass air flow, there are many pressure ratios that can supply enough air. Thus, if the pressure ratio increases, and mass air flow does not, then the output is being more restricted. (it works the same way with a garden hose nozzle)

Of course, in the real world, the "restriction" of compressor output isn't a test valve. It's the physical ability of the engine to induct, react, and exhaust air. That means better "breathing" engines require less pressure and compressor speed to obtain a particular level of mass air flow.

READING TURBO COMPRESSOR MAPS, PART II

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft turbo blog)

Turbo Tip of the Day: "Let it eat" -- Dean Skuza (Former AA/FC racer)

Moving from left to right on our old T66 turbo map, we see the dotted line labled "Surge Limit."

Pressures to the left of the surge limit are not mapped. Why? Because they are both unstable and potentially destructive to the compressor.

Surge amounts to air backing up inside the compressor and fighting to get back out through the entrance (or more properly, the "inducer bore")

To understand surge, imagine air being agitated into a mini-tornado by the compressor impeller (that's the fan blade/meat grinder thing that rotates).

Impeller

When flow out of the turbo is shut off or excessively restricted, the mini-tornado is not properly diffused into steady pressure because it has no place to go. So air being air, it takes the path of least resistance toward a lower pressure -- backing "out through the in door."

These reversals of flow fight the impeller's rotation. The exiting air molecules slam up against other air molecules that the impeller is attempting to induct (draw in). That causes inlet pressures to fluctuate and the impeller's blades to lose efficiency.

Thus, on the left side of the surge limit, the turbo compressor is not doing useful work because the exit flow is too restricted.

Simply put, surge occurs when the attempted pressure ratio is too high for the amount of air consumed by the engine. (Remember, just like with your garden hose or shop compressor, "boost" is not mass air flow. Boost without air flow creates surge)

Surge is most easily found (and heard) when a downstream throttle is slammed shut while the compressor is at speed. Surge sounds like chirping out of the compressor. Blow-off and recirculating valves are often used to combat this form of surge.

Using a turbo that is too large can also produce surge when the boost threshold is lower than an engine's abiliy to induct the compressed charge.

The simple rule is that for your turbo to live, you've got to "let it eat" by avoiding surge.