Polymorphism in C# makes no sense

public interface ICar
{
    string Name { get; }
    IEngine Engine { get; }
}

public interface IEngine
{
    string Name { get; }
}

public class Engine: IEngine
{
    public string Name { get; }
    public string EngineClassMethod()
    {
        return "Engine class";
    }
}

public class Car : ICar
{
    public string Name { get; }
    public Engine Engine { get; }
}

Error    CS0738    'Car' does not implement interface member 'ICar.Engine'. 'Car.Engine' cannot implement 'ICar.Engine' because it does not have the matching return type of 'IEngine'