Here's a little more elaboration:

I looked up the autoignition temperature of Hydrogen, it is 1085 degrees Fahrenheit, or 585 Centigrade, or 858 degrees Kelvin.

http://www1.eere.energy.gov/hydrogen...fs/fcm01r0.pdf
(page 26)

http://chemistry.about.com/library/w...empconvert.htm
(Temperature conversion calculator.)

Now, using the "Diesel Cycle" calculator in the middle of this page:
http://members.aol.com/engware/calc3.htm
Starting with 1 atmosphere of pressure, in other words, no turbocharging,
858 degrees Kelvin is reached at about 14.17:1 of adiabatic compression.

With turbocharging, it is reached even sooner.