Tuesday, July 9, 2019

Salt Slush Summer Reading. Chapter one - Turbo & Supercharger Matching part 1

It’s summertime. It’s vacation time. Sun is shining and the asphalt is warm. A perfect time for some Salt Slush summer reading. This time we will expand on our favorite topic, engine boosting. During the summer we will, in a couple of blog post go through the basics of turbo and supercharging without using too much mathematical formulas.
Why boosting?
The main reason for using turbos and superchargers is the low atmospheric pressure on planet earth. 1 bar (14,5psi) absolute pressure can only provide so much torque and power for a given engine displacement. No matter how much you play around with compression ratio, camshafts etc. If you want more than approximately 90hp/litre displacement (or 90hp/61in³) and 110Nm/litre (110Nm/61in³) a boosting device is the way forward. I guess if you read this you are more than likely to be one of them who want more power than a natural aspirated engine can offer. But how to start?

Gasoline engine Turbo and Supercharger matching
First thing to do, is to start thinking about how much power and torque you want and what kind of character you want from your engine. Ignore everything else to start with. 
No matter if you choose turbo or supercharger you need to start with figure out which peak power you want. 
Why? Because peak power relates to a certain airflow needed for a certain power and certain engine. 
Therefore, first step is to specify a certain compressor (or blower) size.

I write for a certain engine, because different engines will need different airflow depending on their configuration, i.e. how the combustion and gas exchange system are designed. If it’s a 2 valve or 4 valve engine, which bore x stroke ratio, compression ratio and combustion system layout (Port Fuel Injection (PFI) or Direct Injection (DI) etc.) and obviously which fuel that will be used.

Since most of today’s tuners are working on PFI engines, I choose to focus this first post on this type.

Example 1: A reasonable modern 4-valve engine PFI engine need approximately 850-900kg/h (31-33 lbs/min) of air to produce 300hp with a modest back pressure and good intercooling. This is not an exact value, but it gives a hint when it comes turbo matching. Obviously, 600hp would need 1700-1800kg/h (62-66 lbs/min) air and so on.
BorgWarner EFR 9180 turbo compressor map.
Then let's move on to a turbo compressor map of a modern turbo, with pressure ratio on y-axis (pressure difference before and after compressor) and mass flow on x-axis. The islands in the map show compressor efficiency and indicates how efficiently the compressor is in each speed-load point.

If we assume a pressure ratio of 2,8 and the air flow from the example above, we would at 95lbs/min get between 863hp and 920hp on premium gasoline fuel (RON 98 or similar). But this theoretical figure depends on several parameters. Among them knock sensitivity, charge air cooling efficiency, engine back pressure, inlet depression, cam timing and so on. 

Obviously, the more efficient an engine run at peak power, the lower boost pressure & air flow it needs for a given power.
Eaton TVS2300 Supercharger map

Lets move on to superchargers. The crank obviously drives a positive displacement roots supercharger like the Eaton TVS, but the matching process is similar to turbo.
You still have to determine which peak power you are looking for to get a supercharger that can deliver the corresponding airflow.

The supercharged engine more works like a natural aspirated engine, since it has no turbine blocking the exhaust, but on the other hand the supercharger takes energy from the crankshaft. 
This means that it all boils down to total engine and boost devices efficiency at peak power. The supercharger advantage is that it is possibility to play around with supercharger pulley ratio without having problem with lag. The basic function of any positive displacement supercharger is to provide more air than the engine can swallow and this, in combination with a direct connection to the crankshaft, eliminate the lag problem that is associated with high power turbo engine. (In coming blog posts we will talk about how to reduce turbo lag). What I mean is, that the penalty for driving around with a too big supercharger is not too severe.

Example 2: If we look at Eaton TVS2300 map above (which is the heart in many OEM applications and as well in Magnuson Supercharger kit) and recalculate to mass flow i.e. kg/h (or lbs/min) we would end up with at least 770hp (with more or less standard emission system and a standard cam)
The Magnuson DI TVS 2650R Supercharger

With this nice picture I end Matching Part 1. Stay tuned for more..    

No comments:

Post a Comment