#### Answer

The diesel engine is more efficient.

#### Work Step by Step

We know that the value of $\gamma$ in air is 1.4.
We use the efficiency of the gas engine found in problem 55:
$e=1-r^{1-\gamma}$
$e=1-8.3^{1-1.4}=.57$
In problem 57, we found that the efficiency of the diesel engine is given by:
$e_{diesel}=1-\frac{r^{1-\gamma}(\alpha^{\gamma}-1)}{(\alpha-1)\gamma}$
Thus, we find:
$e_{diesel}=1-\frac{19^{1-1.4}(2.4^{1.4}-1)}{(2.4-1)(1.4)}=.62$
The diesel engine is more efficient.