Thread: TD5 compression test readings

I would say thats very good.

One I tested recently was in the region of 150-165 across the board and still started ok!!

I would also be interested to know what the actual book figres should be.

P1V1=P2V2

assuming the temp stays the same which is not exactly correct. But if you waited for the gas/air to cool down after compressing it you would get a lower pressure reading than when the gas was still hot.

P2 = P1V1/V2 (V1/V2 = 19.5)

P1 is one atmosphere if we take it that the turbo is not proving boost.

P2 = 1 Atmosphere * 19.5
P2 = 19.5 Atmosphers (use 15 psi as an atmosphere for ease of calc and round up to 20:1 compression ration

P2 = 600 psi therefore 585 psi above ambient pressure. (that is ideal with out any pressure loss through valves or rings)

added after I checked my mental multiplication (15 x 20 = 300 not 600) I got a bit carried away.

1 Atmosphere = 14.696 psi

14.696 * 19.5 = 286.572

286.572 -14.696 = 271.876 psi above one atmosphere.

that makes the 450 psi look a bit high but that might be due to the heat in the gas and once cooled will reduce.

Close Slug Burner
Off course you quoting a STATIC COMPRESSION
In fact with your Compression guage you are measuring "DYNAMIC Compression pressuire .in perfect air
Your little calc will be close enough except you cant disregard the "specific heat rise " which in air will multiply it by a ratio of 1.4
Off course Cam Overlap will radically change the real Dynamic figures , not the the theoretical Static pressures (why I build my engines with a static comp of between 11 and 13 to ! , and let the overlap on the cam reduce real dynamic down to 9.5:1.
286.572 *1.4 = 401 PSI
where γ is the ratio of specific heat for air (approximately 1.4). The values in the table above are derived using this formula. Note that in reality the ratio of specific heats changes with temperature and that significant deviations from adiabatic behavior will occur.

Boyle's Law works for a constant temperature. Charle's Law brings temp in and the combination of the two would allow us to work with both pressure and temprature changes. If we had the temperature of the air in the compression chamber both before and after it was squeezed we could get the exact theoretical value for the pressure.

When it comes to thermodynamics I must have had a mental block or a shocking lecturer so I don't know how the gamma symbol comes into play and the way it is written it looks like it is an exponent so as you multipled by 1.4 I'd assume that it is not the value of gamma.

I did say that there was an incorrect assumption, that of the temperature staying constant. As I don't have any training beyond high school science that I actually remember that would help in this situation I'll bow out.

I have been told by 2 mechanics that over 400psi for a diesel is good. Does anyone have figures for a TD5

Erich

Well Bundalene , your mechanics are right , except id say over 450 would be better .There are just so many parameters/conditions as to what the theoretical Presure actually is that its all rather irrelevant .Next time you Comp check it do it with the Glow plugs fired up and see what that does , then add a little engine oil , then disconnect the main air feed to the plenum/manifold so its not restricted, then do it on a 10 degree day and a 35 degree day , etc etc etc . the figure is 19.5:1 a static CR not the Dynamic CR ,the pressure is irrelavent , how does it perform ?

Hi Erich,

For what it's worth a 300Tdi which also has a 19.5:1 comp ratio.
The book says with a warm motor and apparently the other three injectors in place crank for 10-20 second and it should have 24 bar/348 lbf/in2.
I recently had mine checked, and at 250,000 Km
No. 1 380
No. 2 390
No. 3 280
No. 4 360
I didn't get a fifth one so I can't help you there

Tony

Originally Posted by slug_burner

Boyle's Law works for a constant temperature. Charle's Law brings temp in and the combination of the two would allow us to work with both pressure and temprature changes. If we had the temperature of the air in the compression chamber both before and after it was squeezed we could get the exact theoretical value for the pressure.

When it comes to thermodynamics I must have had a mental block or a shocking lecturer so I don't know how the gamma symbol comes into play and the way it is written it looks like it is an exponent so as you multipled by 1.4 I'd assume that it is not the value of gamma.

I did say that there was an incorrect assumption, that of the temperature staying constant. As I don't have any training beyond high school science that I actually remember that would help in this situation I'll bow out.

Adiabatic compression.

Adiabatic compression using 1.4 for air is for compression fast enough that no heat-transfer occurs out of the gas being compressed, it heats internally and this raises the pressure read above boyles law. It's a very good approximation, but I have heard of people modifying that exponent to account for more heat-loss.

Of course when you attach a compression tester the volume of the tester reduces the compression ratio and the pressures measured.

Adiabatic compression on 19.5:1 when fed 14.7psi atmospheric would be 940psi.
Hitting 450psi from 14.7psi would only need a compression ratio of 11.5:1. With a little extra volume from the tester and a little heat loss it's easy to lose a lot of pressure.