There doesn’t seem to be much details about what is new about this rocket engine. My guess would be it is using solid and instead of liquid fuel which means it is easier to hide the missiles and quickly fire them:
North Korea claimed it successfully tested a powerful new rocket engine on Sunday, signaling defiance as the U.S. secretary of state visited China.
North Korean leader Kim Jong Un observed the test of a new high-thrust engine at dawn at the Sohae satellite launching pad, the state-run Korean Central News Agency reported.
It quoted him as saying the test “declared a new birth of the Juche-based rocket industry.” Juche, or self-reliance, refers to the communist state’s official ideology.
The test occurred as Secretary of State Rex Tillerson was due to meet with Chinese President Xi Jinping in Beijing.
Tillerson is pushing for closer China-U.S. cooperation on dealing with the growing threat from North Korea’s nuclear weapons program.
China was the last stop on his three-country tour that also took him to South Korea and Japan.
Tillerson has suggested President Donald Trump’s administration may adopt a tougher strategy toward the North. In Seoul, he warned that the U.S. would consider pre-emptive military action if the threat reaches a level “that we believe requires action.”
KCNA said the “Korean-style” engine was developed by the Academy of the National Defense Science. [Stars & Stripes]
You can read more at the link.